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PERLSUB(1)	       Perl Programmers	Reference Guide		    PERLSUB(1)

NAME
       perlsub - Perl subroutines

SYNOPSIS
       To declare subroutines:

	   sub NAME;			 # A "forward" declaration.
	   sub NAME(PROTO);		 #  ditto, but with prototypes
	   sub NAME : ATTRS;		 #  with attributes
	   sub NAME(PROTO) : ATTRS;	 #  with attributes and	prototypes

	   sub NAME BLOCK		 # A declaration and a definition.
	   sub NAME(PROTO) BLOCK	 #  ditto, but with prototypes
	   sub NAME : ATTRS BLOCK	 #  with attributes
	   sub NAME(PROTO) : ATTRS BLOCK #  with prototypes and	attributes

	   use feature 'signatures';
	   sub NAME(SIG) BLOCK			  # with signature
	   sub NAME :ATTRS (SIG) BLOCK		  # with signature, attributes
	   sub NAME :prototype(PROTO) (SIG) BLOCK # with signature, prototype

       To define an anonymous subroutine at runtime:

	   $subref = sub BLOCK;			# no proto
	   $subref = sub (PROTO) BLOCK;		# with proto
	   $subref = sub : ATTRS BLOCK;		# with attributes
	   $subref = sub (PROTO) : ATTRS BLOCK;	# with proto and attributes

	   use feature 'signatures';
	   $subref = sub (SIG) BLOCK;		# with signature
	   $subref = sub : ATTRS(SIG) BLOCK;	# with signature, attributes

       To import subroutines:

	   use MODULE qw(NAME1 NAME2 NAME3);

       To call subroutines:

	   NAME(LIST);	  # & is optional with parentheses.
	   NAME	LIST;	  # Parentheses	optional if predeclared/imported.
	   &NAME(LIST);	  # Circumvent prototypes.
	   &NAME;	  # Makes current @_ visible to	called subroutine.

DESCRIPTION
       Like many languages, Perl provides for user-defined subroutines.	 These
       may be located anywhere in the main program, loaded in from other files
       via the "do", "require",	or "use" keywords, or generated	on the fly
       using "eval" or anonymous subroutines.  You can even call a function
       indirectly using	a variable containing its name or a CODE reference.

       The Perl	model for function call	and return values is simple: all
       functions are passed as parameters one single flat list of scalars, and
       all functions likewise return to	their caller one single	flat list of
       scalars.	 Any arrays or hashes in these call and	return lists will
       collapse, losing	their identities--but you may always use pass-by-
       reference instead to avoid this.	 Both call and return lists may
       contain as many or as few scalar	elements as you'd like.	 (Often	a
       function	without	an explicit return statement is	called a subroutine,
       but there's really no difference	from Perl's perspective.)

       Any arguments passed in show up in the array @_.	 (They may also	show
       up in lexical variables introduced by a signature; see "Signatures"
       below.)	Therefore, if you called a function with two arguments,	those
       would be	stored in $_[0]	and $_[1].  The	array @_ is a local array, but
       its elements are	aliases	for the	actual scalar parameters.  In
       particular, if an element $_[0] is updated, the corresponding argument
       is updated (or an error occurs if it is not updatable).	If an argument
       is an array or hash element which did not exist when the	function was
       called, that element is created only when (and if) it is	modified or a
       reference to it is taken.  (Some	earlier	versions of Perl created the
       element whether or not the element was assigned to.)  Assigning to the
       whole array @_ removes that aliasing, and does not update any
       arguments.

       A "return" statement may	be used	to exit	a subroutine, optionally
       specifying the returned value, which will be evaluated in the
       appropriate context (list, scalar, or void) depending on	the context of
       the subroutine call.  If	you specify no return value, the subroutine
       returns an empty	list in	list context, the undefined value in scalar
       context,	or nothing in void context.  If	you return one or more
       aggregates (arrays and hashes), these will be flattened together	into
       one large indistinguishable list.

       If no "return" is found and if the last statement is an expression, its
       value is	returned.  If the last statement is a loop control structure
       like a "foreach"	or a "while", the returned value is unspecified.  The
       empty sub returns the empty list.

       Aside from an experimental facility (see	"Signatures" below), Perl does
       not have	named formal parameters.  In practice all you do is assign to
       a "my()"	list of	these.	Variables that aren't declared to be private
       are global variables.  For gory details on creating private variables,
       see "Private Variables via my()"	and "Temporary Values via local()".
       To create protected environments	for a set of functions in a separate
       package (and probably a separate	file), see "Packages" in perlmod.

       Example:

	   sub max {
	       my $max = shift(@_);
	       foreach $foo (@_) {
		   $max	= $foo if $max < $foo;
	       }
	       return $max;
	   }
	   $bestday = max($mon,$tue,$wed,$thu,$fri);

       Example:

	   # get a line, combining continuation	lines
	   #  that start with whitespace

	   sub get_line	{
	       $thisline = $lookahead;	# global variables!
	       LINE: while (defined($lookahead = <STDIN>)) {
		   if ($lookahead =~ /^[ \t]/) {
		       $thisline .= $lookahead;
		   }
		   else	{
		       last LINE;
		   }
	       }
	       return $thisline;
	   }

	   $lookahead =	<STDIN>;       # get first line
	   while (defined($line	= get_line())) {
	       ...
	   }

       Assigning to a list of private variables	to name	your arguments:

	   sub maybeset	{
	       my($key,	$value)	= @_;
	       $Foo{$key} = $value unless $Foo{$key};
	   }

       Because the assignment copies the values, this also has the effect of
       turning call-by-reference into call-by-value.  Otherwise	a function is
       free to do in-place modifications of @_ and change its caller's values.

	   upcase_in($v1, $v2);	 # this	changes	$v1 and	$v2
	   sub upcase_in {
	       for (@_)	{ tr/a-z/A-Z/ }
	   }

       You aren't allowed to modify constants in this way, of course.  If an
       argument	were actually literal and you tried to change it, you'd	take a
       (presumably fatal) exception.   For example, this won't work:

	   upcase_in("frederick");

       It would	be much	safer if the "upcase_in()" function were written to
       return a	copy of	its parameters instead of changing them	in place:

	   ($v3, $v4) =	upcase($v1, $v2);  # this doesn't change $v1 and $v2
	   sub upcase {
	       return unless defined wantarray;	 # void	context, do nothing
	       my @parms = @_;
	       for (@parms) { tr/a-z/A-Z/ }
	       return wantarray	? @parms : $parms[0];
	   }

       Notice how this (unprototyped) function doesn't care whether it was
       passed real scalars or arrays.  Perl sees all arguments as one big,
       long, flat parameter list in @_.	 This is one area where	Perl's simple
       argument-passing	style shines.  The "upcase()" function would work
       perfectly well without changing the "upcase()" definition even if we
       fed it things like this:

	   @newlist   =	upcase(@list1, @list2);
	   @newlist   =	upcase(	split /:/, $var	);

       Do not, however,	be tempted to do this:

	   (@a,	@b)   =	upcase(@list1, @list2);

       Like the	flattened incoming parameter list, the return list is also
       flattened on return.  So	all you	have managed to	do here	is stored
       everything in @a	and made @b empty.  See	"Pass by Reference" for
       alternatives.

       A subroutine may	be called using	an explicit "&"	prefix.	 The "&" is
       optional	in modern Perl,	as are parentheses if the subroutine has been
       predeclared.  The "&" is	not optional when just naming the subroutine,
       such as when it's used as an argument to	defined() or undef().  Nor is
       it optional when	you want to do an indirect subroutine call with	a
       subroutine name or reference using the "&$subref()" or "&{$subref}()"
       constructs, although the	"$subref->()" notation solves that problem.
       See perlref for more about all that.

       Subroutines may be called recursively.  If a subroutine is called using
       the "&" form, the argument list is optional, and	if omitted, no @_
       array is	set up for the subroutine: the @_ array	at the time of the
       call is visible to subroutine instead.  This is an efficiency mechanism
       that new	users may wish to avoid.

	   &foo(1,2,3);	       # pass three arguments
	   foo(1,2,3);	       # the same

	   foo();	       # pass a	null list
	   &foo();	       # the same

	   &foo;	       # foo() get current args, like foo(@_) !!
	   foo;		       # like foo() IFF	sub foo	predeclared, else "foo"

       Not only	does the "&" form make the argument list optional, it also
       disables	any prototype checking on arguments you	do provide.  This is
       partly for historical reasons, and partly for having a convenient way
       to cheat	if you know what you're	doing.	See "Prototypes" below.

       Since Perl 5.16.0, the "__SUB__"	token is available under "use feature
       'current_sub'" and "use 5.16.0".	 It will evaluate to a reference to
       the currently-running sub, which	allows for recursive calls without
       knowing your subroutine's name.

	   use 5.16.0;
	   my $factorial = sub {
	     my	($x) = @_;
	     return 1 if $x == 1;
	     return($x * __SUB__->( $x - 1 ) );
	   };

       The behavior of "__SUB__" within	a regex	code block (such as
       "/(?{...})/") is	subject	to change.

       Subroutines whose names are in all upper	case are reserved to the Perl
       core, as	are modules whose names	are in all lower case.	A subroutine
       in all capitals is a loosely-held convention meaning it will be called
       indirectly by the run-time system itself, usually due to	a triggered
       event.  Subroutines whose name start with a left	parenthesis are	also
       reserved	the same way.  The following is	a list of some subroutines
       that currently do special, pre-defined things.

       documented later	in this	document
	   "AUTOLOAD"

       documented in perlmod
	   "CLONE", "CLONE_SKIP"

       documented in perlobj
	   "DESTROY", "DOES"

       documented in perltie
	   "BINMODE", "CLEAR", "CLOSE",	"DELETE", "DESTROY", "EOF", "EXISTS",
	   "EXTEND", "FETCH", "FETCHSIZE", "FILENO", "FIRSTKEY", "GETC",
	   "NEXTKEY", "OPEN", "POP", "PRINT", "PRINTF",	"PUSH",	"READ",
	   "READLINE", "SCALAR", "SEEK", "SHIFT", "SPLICE", "STORE",
	   "STORESIZE",	"TELL",	"TIEARRAY", "TIEHANDLE", "TIEHASH",
	   "TIESCALAR",	"UNSHIFT", "UNTIE", "WRITE"

       documented in PerlIO::via
	   "BINMODE", "CLEARERR", "CLOSE", "EOF", "ERROR", "FDOPEN", "FILENO",
	   "FILL", "FLUSH", "OPEN", "POPPED", "PUSHED",	"READ",	"SEEK",
	   "SETLINEBUF", "SYSOPEN", "TELL", "UNREAD", "UTF8", "WRITE"

       documented in perlfunc
	   "import" , "unimport" , "INC"

       documented in UNIVERSAL
	   "VERSION"

       documented in perldebguts
	   "DB::DB", "DB::sub",	"DB::lsub", "DB::goto",	"DB::postponed"

       undocumented, used internally by	the overload feature
	   any starting	with "("

       The "BEGIN", "UNITCHECK", "CHECK", "INIT" and "END" subroutines are not
       so much subroutines as named special code blocks, of which you can have
       more than one in	a package, and which you can not call explicitly.  See
       "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod

   Signatures
       WARNING:	Subroutine signatures are experimental.	 The feature may be
       modified	or removed in future versions of Perl.

       Perl has	an experimental	facility to allow a subroutine's formal
       parameters to be	introduced by special syntax, separate from the
       procedural code of the subroutine body.	The formal parameter list is
       known as	a signature.  The facility must	be enabled first by a
       pragmatic declaration, "use feature 'signatures'", and it will produce
       a warning unless	the "experimental::signatures" warnings	category is
       disabled.

       The signature is	part of	a subroutine's body.  Normally the body	of a
       subroutine is simply a braced block of code, but	when using a
       signature, the signature	is a parenthesised list	that goes immediately
       before the block, after any name	or attributes.

       For example,

	   sub foo :lvalue ($a,	$b = 1,	@c) { .... }

       The signature declares lexical variables	that are in scope for the
       block.  When the	subroutine is called, the signature takes control
       first.  It populates the	signature variables from the list of arguments
       that were passed.  If the argument list doesn't meet the	requirements
       of the signature, then it will throw an exception.  When	the signature
       processing is complete, control passes to the block.

       Positional parameters are handled by simply naming scalar variables in
       the signature.  For example,

	   sub foo ($left, $right) {
	       return $left + $right;
	   }

       takes two positional parameters,	which must be filled at	runtime	by two
       arguments.  By default the parameters are mandatory, and	it is not
       permitted to pass more arguments	than expected.	So the above is
       equivalent to

	   sub foo {
	       die "Too	many arguments for subroutine" unless @_ <= 2;
	       die "Too	few arguments for subroutine" unless @_	>= 2;
	       my $left	= $_[0];
	       my $right = $_[1];
	       return $left + $right;
	   }

       An argument can be ignored by omitting the main part of the name	from a
       parameter declaration, leaving just a bare "$" sigil.  For example,

	   sub foo ($first, $, $third) {
	       return "first=$first, third=$third";
	   }

       Although	the ignored argument doesn't go	into a variable, it is still
       mandatory for the caller	to pass	it.

       A positional parameter is made optional by giving a default value,
       separated from the parameter name by "=":

	   sub foo ($left, $right = 0) {
	       return $left + $right;
	   }

       The above subroutine may	be called with either one or two arguments.
       The default value expression is evaluated when the subroutine is
       called, so it may provide different default values for different	calls.
       It is only evaluated if the argument was	actually omitted from the
       call.  For example,

	   my $auto_id = 0;
	   sub foo ($thing, $id	= $auto_id++) {
	       print "$thing has ID $id";
	   }

       automatically assigns distinct sequential IDs to	things for which no ID
       was supplied by the caller.  A default value expression may also	refer
       to parameters earlier in	the signature, making the default for one
       parameter vary according	to the earlier parameters.  For	example,

	   sub foo ($first_name, $surname, $nickname = $first_name) {
	       print "$first_name $surname is known as \"$nickname\"";
	   }

       An optional parameter can be nameless just like a mandatory parameter.
       For example,

	   sub foo ($thing, $ =	1) {
	       print $thing;
	   }

       The parameter's default value will still	be evaluated if	the
       corresponding argument isn't supplied, even though the value won't be
       stored anywhere.	 This is in case evaluating it has important side
       effects.	 However, it will be evaluated in void context,	so if it
       doesn't have side effects and is	not trivial it will generate a warning
       if the "void" warning category is enabled.  If a	nameless optional
       parameter's default value is not	important, it may be omitted just as
       the parameter's name was:

	   sub foo ($thing, $=)	{
	       print $thing;
	   }

       Optional	positional parameters must come	after all mandatory positional
       parameters.  (If	there are no mandatory positional parameters then an
       optional	positional parameters can be the first thing in	the
       signature.)  If there are multiple optional positional parameters and
       not enough arguments are	supplied to fill them all, they	will be	filled
       from left to right.

       After positional	parameters, additional arguments may be	captured in a
       slurpy parameter.  The simplest form of this is just an array variable:

	   sub foo ($filter, @inputs) {
	       print $filter->($_) foreach @inputs;
	   }

       With a slurpy parameter in the signature, there is no upper limit on
       how many	arguments may be passed.  A slurpy array parameter may be
       nameless	just like a positional parameter, in which case	its only
       effect is to turn off the argument limit	that would otherwise apply:

	   sub foo ($thing, @) {
	       print $thing;
	   }

       A slurpy	parameter may instead be a hash, in which case the arguments
       available to it are interpreted as alternating keys and values.	There
       must be as many keys as values: if there	is an odd argument then	an
       exception will be thrown.  Keys will be stringified, and	if there are
       duplicates then the later instance takes	precedence over	the earlier,
       as with standard	hash construction.

	   sub foo ($filter, %inputs) {
	       print $filter->($_, $inputs{$_})	foreach	sort keys %inputs;
	   }

       A slurpy	hash parameter may be nameless just like other kinds of
       parameter.  It still insists that the number of arguments available to
       it be even, even	though they're not being put into a variable.

	   sub foo ($thing, %) {
	       print $thing;
	   }

       A slurpy	parameter, either array	or hash, must be the last thing	in the
       signature.  It may follow mandatory and optional	positional parameters;
       it may also be the only thing in	the signature.	Slurpy parameters
       cannot have default values: if no arguments are supplied	for them then
       you get an empty	array or empty hash.

       A signature may be entirely empty, in which case	all it does is check
       that the	caller passed no arguments:

	   sub foo () {
	       return 123;
	   }

       When using a signature, the arguments are still available in the
       special array variable @_, in addition to the lexical variables of the
       signature.  There is a difference between the two ways of accessing the
       arguments: @_ aliases the arguments, but	the signature variables	get
       copies of the arguments.	 So writing to a signature variable only
       changes that variable, and has no effect	on the caller's	variables, but
       writing to an element of	@_ modifies whatever the caller	used to	supply
       that argument.

       There is	a potential syntactic ambiguity	between	signatures and
       prototypes (see "Prototypes"), because both start with an opening
       parenthesis and both can	appear in some of the same places, such	as
       just after the name in a	subroutine declaration.	 For historical
       reasons,	when signatures	are not	enabled, any opening parenthesis in
       such a context will trigger very	forgiving prototype parsing.  Most
       signatures will be interpreted as prototypes in those circumstances,
       but won't be valid prototypes.  (A valid	prototype cannot contain any
       alphabetic character.)  This will lead to somewhat confusing error
       messages.

       To avoid	ambiguity, when	signatures are enabled the special syntax for
       prototypes is disabled.	There is no attempt to guess whether a
       parenthesised group was intended	to be a	prototype or a signature.  To
       give a subroutine a prototype under these circumstances,	use a
       prototype attribute.  For example,

	   sub foo :prototype($) { $_[0] }

       It is entirely possible for a subroutine	to have	both a prototype and a
       signature.  They	do different jobs: the prototype affects compilation
       of calls	to the subroutine, and the signature puts argument values into
       lexical variables at runtime.  You can therefore	write

	   sub foo :prototype($$) ($left, $right) {
	       return $left + $right;
	   }

       The prototype attribute,	and any	other attributes, must come before the
       signature.  The signature always	immediately precedes the block of the
       subroutine's body.

   Private Variables via my()
       Synopsis:

	   my $foo;	       # declare $foo lexically	local
	   my (@wid, %get);    # declare list of variables local
	   my $foo = "flurp";  # declare $foo lexical, and init	it
	   my @oof = @bar;     # declare @oof lexical, and init	it
	   my $x : Foo = $y;   # similar, with an attribute applied

       WARNING:	The use	of attribute lists on "my" declarations	is still
       evolving.  The current semantics	and interface are subject to change.
       See attributes and Attribute::Handlers.

       The "my"	operator declares the listed variables to be lexically
       confined	to the enclosing block,	conditional
       ("if"/"unless"/"elsif"/"else"), loop
       ("for"/"foreach"/"while"/"until"/"continue"), subroutine, "eval", or
       "do"/"require"/"use"'d file.  If	more than one value is listed, the
       list must be placed in parentheses.  All	listed elements	must be	legal
       lvalues.	 Only alphanumeric identifiers may be lexically
       scoped--magical built-ins like $/ must currently	be "local"ized with
       "local" instead.

       Unlike dynamic variables	created	by the "local" operator, lexical
       variables declared with "my" are	totally	hidden from the	outside	world,
       including any called subroutines.  This is true if it's the same
       subroutine called from itself or	elsewhere--every call gets its own
       copy.

       This doesn't mean that a	"my" variable declared in a statically
       enclosing lexical scope would be	invisible.  Only dynamic scopes	are
       cut off.	  For example, the "bumpx()" function below has	access to the
       lexical $x variable because both	the "my" and the "sub" occurred	at the
       same scope, presumably file scope.

	   my $x = 10;
	   sub bumpx { $x++ }

       An "eval()", however, can see lexical variables of the scope it is
       being evaluated in, so long as the names	aren't hidden by declarations
       within the "eval()" itself.  See	perlref.

       The parameter list to my() may be assigned to if	desired, which allows
       you to initialize your variables.  (If no initializer is	given for a
       particular variable, it is created with the undefined value.)  Commonly
       this is used to name input parameters to	a subroutine.  Examples:

	   $arg	= "fred";	 # "global" variable
	   $n =	cube_root(27);
	   print "$arg thinks the root is $n\n";
	fred thinks the	root is	3

	   sub cube_root {
	       my $arg = shift;	 # name	doesn't	matter
	       $arg **=	1/3;
	       return $arg;
	   }

       The "my"	is simply a modifier on	something you might assign to.	So
       when you	do assign to variables in its argument list, "my" doesn't
       change whether those variables are viewed as a scalar or	an array.  So

	   my ($foo) = <STDIN>;		       # WRONG?
	   my @FOO = <STDIN>;

       both supply a list context to the right-hand side, while

	   my $foo = <STDIN>;

       supplies	a scalar context.  But the following declares only one
       variable:

	   my $foo, $bar = 1;		       # WRONG

       That has	the same effect	as

	   my $foo;
	   $bar	= 1;

       The declared variable is	not introduced (is not visible)	until after
       the current statement.  Thus,

	   my $x = $x;

       can be used to initialize a new $x with the value of the	old $x,	and
       the expression

	   my $x = 123 and $x == 123

       is false	unless the old $x happened to have the value 123.

       Lexical scopes of control structures are	not bounded precisely by the
       braces that delimit their controlled blocks; control expressions	are
       part of that scope, too.	 Thus in the loop

	   while (my $line = <>) {
	       $line = lc $line;
	   } continue {
	       print $line;
	   }

       the scope of $line extends from its declaration throughout the rest of
       the loop	construct (including the "continue" clause), but not beyond
       it.  Similarly, in the conditional

	   if ((my $answer = <STDIN>) =~ /^yes$/i) {
	       user_agrees();
	   } elsif ($answer =~ /^no$/i)	{
	       user_disagrees();
	   } else {
	       chomp $answer;
	       die "'$answer' is neither 'yes' nor 'no'";
	   }

       the scope of $answer extends from its declaration through the rest of
       that conditional, including any "elsif" and "else" clauses, but not
       beyond it.  See "Simple Statements" in perlsyn for information on the
       scope of	variables in statements	with modifiers.

       The "foreach" loop defaults to scoping its index	variable dynamically
       in the manner of	"local".  However, if the index	variable is prefixed
       with the	keyword	"my", or if there is already a lexical by that name in
       scope, then a new lexical is created instead.  Thus in the loop

	   for my $i (1, 2, 3) {
	       some_function();
	   }

       the scope of $i extends to the end of the loop, but not beyond it,
       rendering the value of $i inaccessible within "some_function()".

       Some users may wish to encourage	the use	of lexically scoped variables.
       As an aid to catching implicit uses to package variables, which are
       always global, if you say

	   use strict 'vars';

       then any	variable mentioned from	there to the end of the	enclosing
       block must either refer to a lexical variable, be predeclared via "our"
       or "use vars", or else must be fully qualified with the package name.
       A compilation error results otherwise.  An inner	block may countermand
       this with "no strict 'vars'".

       A "my" has both a compile-time and a run-time effect.  At compile time,
       the compiler takes notice of it.	 The principal usefulness of this is
       to quiet	"use strict 'vars'", but it is also essential for generation
       of closures as detailed in perlref.  Actual initialization is delayed
       until run time, though, so it gets executed at the appropriate time,
       such as each time through a loop, for example.

       Variables declared with "my" are	not part of any	package	and are
       therefore never fully qualified with the	package	name.  In particular,
       you're not allowed to try to make a package variable (or	other global)
       lexical:

	   my $pack::var;      # ERROR!	 Illegal syntax

       In fact,	a dynamic variable (also known as package or global variables)
       are still accessible using the fully qualified "::" notation even while
       a lexical of the	same name is also visible:

	   package main;
	   local $x = 10;
	   my	 $x = 20;
	   print "$x and $::x\n";

       That will print out 20 and 10.

       You may declare "my" variables at the outermost scope of	a file to hide
       any such	identifiers from the world outside that	file.  This is similar
       in spirit to C's	static variables when they are used at the file	level.
       To do this with a subroutine requires the use of	a closure (an
       anonymous function that accesses	enclosing lexicals).  If you want to
       create a	private	subroutine that	cannot be called from outside that
       block, it can declare a lexical variable	containing an anonymous	sub
       reference:

	   my $secret_version =	'1.001-beta';
	   my $secret_sub = sub	{ print	$secret_version	};
	   &$secret_sub();

       As long as the reference	is never returned by any function within the
       module, no outside module can see the subroutine, because its name is
       not in any package's symbol table.  Remember that it's not REALLY
       called $some_pack::secret_version or anything; it's just
       $secret_version,	unqualified and	unqualifiable.

       This does not work with object methods, however;	all object methods
       have to be in the symbol	table of some package to be found.  See
       "Function Templates" in perlref for something of	a work-around to this.

   Persistent Private Variables
       There are two ways to build persistent private variables	in Perl	5.10.
       First, you can simply use the "state" feature.  Or, you can use
       closures, if you	want to	stay compatible	with releases older than 5.10.

       Persistent variables via	state()

       Beginning with Perl 5.10.0, you can declare variables with the "state"
       keyword in place	of "my".  For that to work, though, you	must have
       enabled that feature beforehand,	either by using	the "feature" pragma,
       or by using "-E"	on one-liners (see feature).  Beginning	with Perl
       5.16, the "CORE::state" form does not require the "feature" pragma.

       The "state" keyword creates a lexical variable (following the same
       scoping rules as	"my") that persists from one subroutine	call to	the
       next.  If a state variable resides inside an anonymous subroutine, then
       each copy of the	subroutine has its own copy of the state variable.
       However,	the value of the state variable	will still persist between
       calls to	the same copy of the anonymous subroutine.  (Don't forget that
       "sub { ... }" creates a new subroutine each time	it is executed.)

       For example, the	following code maintains a private counter,
       incremented each	time the gimme_another() function is called:

	   use feature 'state';
	   sub gimme_another { state $x; return	++$x }

       And this	example	uses anonymous subroutines to create separate
       counters:

	   use feature 'state';
	   sub create_counter {
	       return sub { state $x; return ++$x }
	   }

       Also, since $x is lexical, it can't be reached or modified by any Perl
       code outside.

       When combined with variable declaration,	simple assignment to "state"
       variables (as in	"state $x = 42") is executed only the first time.
       When such statements are	evaluated subsequent times, the	assignment is
       ignored.	 The behavior of assignment to "state" declarations where the
       left hand side of the assignment	involves any parentheses is currently
       undefined.

       Persistent variables with closures

       Just because a lexical variable is lexically (also called statically)
       scoped to its enclosing block, "eval", or "do" FILE, this doesn't mean
       that within a function it works like a C	static.	 It normally works
       more like a C auto, but with implicit garbage collection.

       Unlike local variables in C or C++, Perl's lexical variables don't
       necessarily get recycled	just because their scope has exited.  If
       something more permanent	is still aware of the lexical, it will stick
       around.	So long	as something else references a lexical,	that lexical
       won't be	freed--which is	as it should be.  You wouldn't want memory
       being free until	you were done using it,	or kept	around once you	were
       done.  Automatic	garbage	collection takes care of this for you.

       This means that you can pass back or save away references to lexical
       variables, whereas to return a pointer to a C auto is a grave error.
       It also gives us	a way to simulate C's function statics.	 Here's	a
       mechanism for giving a function private variables with both lexical
       scoping and a static lifetime.  If you do want to create	something like
       C's static variables, just enclose the whole function in	an extra
       block, and put the static variable outside the function but in the
       block.

	   {
	       my $secret_val =	0;
	       sub gimme_another {
		   return ++$secret_val;
	       }
	   }
	   # $secret_val now becomes unreachable by the	outside
	   # world, but	retains	its value between calls	to gimme_another

       If this function	is being sourced in from a separate file via "require"
       or "use", then this is probably just fine.  If it's all in the main
       program,	you'll need to arrange for the "my" to be executed early,
       either by putting the whole block above your main program, or more
       likely, placing merely a	"BEGIN"	code block around it to	make sure it
       gets executed before your program starts	to run:

	   BEGIN {
	       my $secret_val =	0;
	       sub gimme_another {
		   return ++$secret_val;
	       }
	   }

       See "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod about the
       special triggered code blocks, "BEGIN", "UNITCHECK", "CHECK", "INIT"
       and "END".

       If declared at the outermost scope (the file scope), then lexicals work
       somewhat	like C's file statics.	They are available to all functions in
       that same file declared below them, but are inaccessible	from outside
       that file.  This	strategy is sometimes used in modules to create
       private variables that the whole	module can see.

   Temporary Values via	local()
       WARNING:	In general, you	should be using	"my" instead of	"local",
       because it's faster and safer.  Exceptions to this include the global
       punctuation variables, global filehandles and formats, and direct
       manipulation of the Perl	symbol table itself.  "local" is mostly	used
       when the	current	value of a variable must be visible to called
       subroutines.

       Synopsis:

	   # localization of values

	   local $foo;		      #	make $foo dynamically local
	   local (@wid,	%get);	      #	make list of variables local
	   local $foo =	"flurp";      #	make $foo dynamic, and init it
	   local @oof =	@bar;	      #	make @oof dynamic, and init it

	   local $hash{key} = "val";  #	sets a local value for this hash entry
	   delete local	$hash{key};   #	delete this entry for the current block
	   local ($cond	? $v1 :	$v2); #	several	types of lvalues support
				      #	localization

	   # localization of symbols

	   local *FH;		      #	localize $FH, @FH, %FH,	&FH  ...
	   local *merlyn = *randal;   #	now $merlyn is really $randal, plus
				      #	    @merlyn is really @randal, etc
	   local *merlyn = 'randal';  #	SAME THING: promote 'randal' to	*randal
	   local *merlyn = \$randal;  #	just alias $merlyn, not	@merlyn	etc

       A "local" modifies its listed variables to be "local" to	the enclosing
       block, "eval", or "do FILE"--and	to any subroutine called from within
       that block.  A "local" just gives temporary values to global (meaning
       package)	variables.  It does not	create a local variable.  This is
       known as	dynamic	scoping.  Lexical scoping is done with "my", which
       works more like C's auto	declarations.

       Some types of lvalues can be localized as well: hash and	array elements
       and slices, conditionals	(provided that their result is always
       localizable), and symbolic references.  As for simple variables,	this
       creates new, dynamically	scoped values.

       If more than one	variable or expression is given	to "local", they must
       be placed in parentheses.  This operator	works by saving	the current
       values of those variables in its	argument list on a hidden stack	and
       restoring them upon exiting the block, subroutine, or eval.  This means
       that called subroutines can also	reference the local variable, but not
       the global one.	The argument list may be assigned to if	desired, which
       allows you to initialize	your local variables.  (If no initializer is
       given for a particular variable,	it is created with an undefined
       value.)

       Because "local" is a run-time operator, it gets executed	each time
       through a loop.	Consequently, it's more	efficient to localize your
       variables outside the loop.

       Grammatical note	on local()

       A "local" is simply a modifier on an lvalue expression.	When you
       assign to a "local"ized variable, the "local" doesn't change whether
       its list	is viewed as a scalar or an array.  So

	   local($foo) = <STDIN>;
	   local @FOO =	<STDIN>;

       both supply a list context to the right-hand side, while

	   local $foo =	<STDIN>;

       supplies	a scalar context.

       Localization of special variables

       If you localize a special variable, you'll be giving a new value	to it,
       but its magic won't go away.  That means	that all side-effects related
       to this magic still work	with the localized value.

       This feature allows code	like this to work :

	   # Read the whole contents of	FILE in	$slurp
	   { local $/ =	undef; $slurp =	<FILE>;	}

       Note, however, that this	restricts localization of some values ;	for
       example,	the following statement	dies, as of perl 5.10.0, with an error
       Modification of a read-only value attempted, because the	$1 variable is
       magical and read-only :

	   local $1 = 2;

       One exception is	the default scalar variable: starting with perl	5.14
       "local($_)" will	always strip all magic from $_,	to make	it possible to
       safely reuse $_ in a subroutine.

       WARNING:	Localization of	tied arrays and	hashes does not	currently work
       as described.  This will	be fixed in a future release of	Perl; in the
       meantime, avoid code that relies	on any particular behavior of
       localising tied arrays or hashes	(localising individual elements	is
       still okay).  See "Localising Tied Arrays and Hashes Is Broken" in
       perl58delta for more details.

       Localization of globs

       The construct

	   local *name;

       creates a whole new symbol table	entry for the glob "name" in the
       current package.	 That means that all variables in its glob slot
       ($name, @name, %name, &name, and	the "name" filehandle) are dynamically
       reset.

       This implies, among other things, that any magic	eventually carried by
       those variables is locally lost.	 In other words, saying	"local */"
       will not	have any effect	on the internal	value of the input record
       separator.

       Localization of elements	of composite types

       It's also worth taking a	moment to explain what happens when you
       "local"ize a member of a	composite type (i.e. an	array or hash
       element).  In this case,	the element is "local"ized by name.  This
       means that when the scope of the	"local()" ends,	the saved value	will
       be restored to the hash element whose key was named in the "local()",
       or the array element whose index	was named in the "local()".  If	that
       element was deleted while the "local()" was in effect (e.g. by a
       "delete()" from a hash or a "shift()" of	an array), it will spring back
       into existence, possibly	extending an array and filling in the skipped
       elements	with "undef".  For instance, if	you say

	   %hash = ( 'This' => 'is', 'a' => 'test' );
	   @ary	 = ( 0..5 );
	   {
		local($ary[5]) = 6;
		local($hash{'a'}) = 'drill';
		while (my $e = pop(@ary)) {
		    print "$e .	. .\n";
		    last unless	$e > 3;
		}
		if (@ary) {
		    $hash{'only	a'} = 'test';
		    delete $hash{'a'};
		}
	   }
	   print join('	', map { "$_ $hash{$_}"	} sort keys %hash),".\n";
	   print "The array has	",scalar(@ary)," elements: ",
		 join(', ', map	{ defined $_ ? $_ : 'undef' } @ary),"\n";

       Perl will print

	   6 . . .
	   4 . . .
	   3 . . .
	   This	is a test only a test.
	   The array has 6 elements: 0,	1, 2, undef, undef, 5

       The behavior of local() on non-existent members of composite types is
       subject to change in future.

       Localized deletion of elements of composite types

       You can use the "delete local $array[$idx]" and "delete local
       $hash{key}" constructs to delete	a composite type entry for the current
       block and restore it when it ends.  They	return the array/hash value
       before the localization,	which means that they are respectively
       equivalent to

	   do {
	       my $val = $array[$idx];
	       local  $array[$idx];
	       delete $array[$idx];
	       $val
	   }

       and

	   do {
	       my $val = $hash{key};
	       local  $hash{key};
	       delete $hash{key};
	       $val
	   }

       except that for those the "local" is scoped to the "do" block.  Slices
       are also	accepted.

	   my %hash = (
	    a => [ 7, 8, 9 ],
	    b => 1,
	   )

	   {
	    my $a = delete local $hash{a};
	    # $a is [ 7, 8, 9 ]
	    # %hash is (b => 1)

	    {
	     my	@nums =	delete local @$a[0, 2]
	     # @nums is	(7, 9)
	     # $a is [ undef, 8	]

	     $a[0] = 999; # will be erased when	the scope ends
	    }
	    # $a is back to [ 7, 8, 9 ]

	   }
	   # %hash is back to its original state

   Lvalue subroutines
       It is possible to return	a modifiable value from	a subroutine.  To do
       this, you have to declare the subroutine	to return an lvalue.

	   my $val;
	   sub canmod :	lvalue {
	       $val;  #	or:  return $val;
	   }
	   sub nomod {
	       $val;
	   }

	   canmod() = 5;   # assigns to	$val
	   nomod()  = 5;   # ERROR

       The scalar/list context for the subroutine and for the right-hand side
       of assignment is	determined as if the subroutine	call is	replaced by a
       scalar.	For example, consider:

	   data(2,3) = get_data(3,4);

       Both subroutines	here are called	in a scalar context, while in:

	   (data(2,3)) = get_data(3,4);

       and in:

	   (data(2),data(3)) = get_data(3,4);

       all the subroutines are called in a list	context.

       Lvalue subroutines are convenient, but you have to keep in mind that,
       when used with objects, they may	violate	encapsulation.	A normal
       mutator can check the supplied argument before setting the attribute it
       is protecting, an lvalue	subroutine cannot.  If you require any special
       processing when storing and retrieving the values, consider using the
       CPAN module Sentinel or something similar.

   Lexical Subroutines
       Beginning with Perl 5.18, you can declare a private subroutine with
       "my" or "state".	 As with state variables, the "state" keyword is only
       available under "use feature 'state'" or	"use 5.010" or higher.

       Prior to	Perl 5.26, lexical subroutines were deemed experimental	and
       were available only under the "use feature 'lexical_subs'" pragma.
       They also produced a warning unless the "experimental::lexical_subs"
       warnings	category was disabled.

       These subroutines are only visible within the block in which they are
       declared, and only after	that declaration:

	   # Include these two lines if	your code is intended to run under Perl
	   # versions earlier than 5.26.
	   no warnings "experimental::lexical_subs";
	   use feature 'lexical_subs';

	   foo();	       # calls the package/global subroutine
	   state sub foo {
	       foo();	       # also calls the	package	subroutine
	   }
	   foo();	       # calls "state" sub
	   my $ref = \&foo;    # take a	reference to "state" sub

	   my sub bar {	... }
	   bar();	       # calls "my" sub

       You can't (directly) write a recursive lexical subroutine:

	   # WRONG
	   my sub baz {
	       baz();
	   }

       This example fails because "baz()" refers to the	package/global
       subroutine "baz", not the lexical subroutine currently being defined.

       The solution is to use "__SUB__":

	   my sub baz {
	       __SUB__->();    # calls itself
	   }

       It is possible to predeclare a lexical subroutine.  The "sub foo	{...}"
       subroutine definition syntax respects any previous "my sub;" or "state
       sub;" declaration.  Using this to define	recursive subroutines is a bad
       idea, however:

	   my sub baz;	       # predeclaration
	   sub baz {	       # define	the "my" sub
	       baz();	       # WRONG:	calls itself, but leaks	memory
	   }

       Just like "my $f; $f = sub { $f->() }", this example leaks memory.  The
       name "baz" is a reference to the	subroutine, and	the subroutine uses
       the name	"baz"; they keep each other alive (see "Circular References"
       in perlref).

       "state sub" vs "my sub"

       What is the difference between "state" subs and "my" subs?  Each	time
       that execution enters a block when "my" subs are	declared, a new	copy
       of each sub is created.	"State"	subroutines persist from one execution
       of the containing block to the next.

       So, in general, "state" subroutines are faster.	But "my" subs are
       necessary if you	want to	create closures:

	   sub whatever	{
	       my $x = shift;
	       my sub inner {
		   ... do something with $x ...
	       }
	       inner();
	   }

       In this example,	a new $x is created when "whatever" is called, and
       also a new "inner", which can see the new $x.  A	"state"	sub will only
       see the $x from the first call to "whatever".

       "our" subroutines

       Like "our $variable", "our sub" creates a lexical alias to the package
       subroutine of the same name.

       The two main uses for this are to switch	back to	using the package sub
       inside an inner scope:

	   sub foo { ... }

	   sub bar {
	       my sub foo { ...	}
	       {
		   # need to use the outer foo here
		   our sub foo;
		   foo();
	       }
	   }

       and to make a subroutine	visible	to other packages in the same scope:

	   package MySneakyModule;

	   our sub do_something	{ ... }

	   sub do_something_with_caller	{
	       package DB;
	       () = caller 1;	       # sets @DB::args
	       do_something(@args);    # uses MySneakyModule::do_something
	   }

   Passing Symbol Table	Entries	(typeglobs)
       WARNING:	The mechanism described	in this	section	was originally the
       only way	to simulate pass-by-reference in older versions	of Perl.
       While it	still works fine in modern versions, the new reference
       mechanism is generally easier to	work with.  See	below.

       Sometimes you don't want	to pass	the value of an	array to a subroutine
       but rather the name of it, so that the subroutine can modify the	global
       copy of it rather than working with a local copy.  In perl you can
       refer to	all objects of a particular name by prefixing the name with a
       star: *foo.  This is often known	as a "typeglob", because the star on
       the front can be	thought	of as a	wildcard match for all the funny
       prefix characters on variables and subroutines and such.

       When evaluated, the typeglob produces a scalar value that represents
       all the objects of that name, including any filehandle, format, or
       subroutine.  When assigned to, it causes	the name mentioned to refer to
       whatever	"*" value was assigned to it.  Example:

	   sub doubleary {
	       local(*someary) = @_;
	       foreach $elem (@someary)	{
		   $elem *= 2;
	       }
	   }
	   doubleary(*foo);
	   doubleary(*bar);

       Scalars are already passed by reference,	so you can modify scalar
       arguments without using this mechanism by referring explicitly to $_[0]
       etc.  You can modify all	the elements of	an array by passing all	the
       elements	as scalars, but	you have to use	the "*"	mechanism (or the
       equivalent reference mechanism) to "push", "pop", or change the size of
       an array.  It will certainly be faster to pass the typeglob (or
       reference).

       Even if you don't want to modify	an array, this mechanism is useful for
       passing multiple	arrays in a single LIST, because normally the LIST
       mechanism will merge all	the array values so that you can't extract out
       the individual arrays.  For more	on typeglobs, see "Typeglobs and
       Filehandles" in perldata.

   When	to Still Use local()
       Despite the existence of	"my", there are	still three places where the
       "local" operator	still shines.  In fact,	in these three places, you
       must use	"local"	instead	of "my".

       1.  You need to give a global variable a	temporary value, especially
	   $_.

	   The global variables, like @ARGV or the punctuation variables, must
	   be "local"ized with "local()".  This	block reads in /etc/motd, and
	   splits it up	into chunks separated by lines of equal	signs, which
	   are placed in @Fields.

	       {
		   local @ARGV = ("/etc/motd");
		   local $/ = undef;
		   local $_ = <>;
		   @Fields = split /^\s*=+\s*$/;
	       }

	   It particular, it's important to "local"ize $_ in any routine that
	   assigns to it.  Look	out for	implicit assignments in	"while"
	   conditionals.

       2.  You need to create a	local file or directory	handle or a local
	   function.

	   A function that needs a filehandle of its own must use "local()" on
	   a complete typeglob.	  This can be used to create new symbol	table
	   entries:

	       sub ioqueue {
		   local  (*READER, *WRITER);	 # not my!
		   pipe	   (READER,  WRITER)	 or die	"pipe: $!";
		   return (*READER, *WRITER);
	       }
	       ($head, $tail) =	ioqueue();

	   See the Symbol module for a way to create anonymous symbol table
	   entries.

	   Because assignment of a reference to	a typeglob creates an alias,
	   this	can be used to create what is effectively a local function, or
	   at least, a local alias.

	       {
		   local *grow = \&shrink; # only until	this block exits
		   grow();		  # really calls shrink()
		   move();		  # if move() grow()s, it shrink()s too
	       }
	       grow();			  # get	the real grow()	again

	   See "Function Templates" in perlref for more	about manipulating
	   functions by	name in	this way.

       3.  You want to temporarily change just one element of an array or
	   hash.

	   You can "local"ize just one element of an aggregate.	 Usually this
	   is done on dynamics:

	       {
		   local $SIG{INT} = 'IGNORE';
		   funct();			       # uninterruptible
	       }
	       # interruptibility automatically	restored here

	   But it also works on	lexically declared aggregates.

   Pass	by Reference
       If you want to pass more	than one array or hash into a function--or
       return them from	it--and	have them maintain their integrity, then
       you're going to have to use an explicit pass-by-reference.  Before you
       do that,	you need to understand references as detailed in perlref.
       This section may	not make much sense to you otherwise.

       Here are	a few simple examples.	First, let's pass in several arrays to
       a function and have it "pop" all	of then, returning a new list of all
       their former last elements:

	   @tailings = popmany ( \@a, \@b, \@c,	\@d );

	   sub popmany {
	       my $aref;
	       my @retlist;
	       foreach $aref ( @_ ) {
		   push	@retlist, pop @$aref;
	       }
	       return @retlist;
	   }

       Here's how you might write a function that returns a list of keys
       occurring in all	the hashes passed to it:

	   @common = inter( \%foo, \%bar, \%joe	);
	   sub inter {
	       my ($k, $href, %seen); #	locals
	       foreach $href (@_) {
		   while ( $k =	each %$href ) {
		       $seen{$k}++;
		   }
	       }
	       return grep { $seen{$_} == @_ } keys %seen;
	   }

       So far, we're using just	the normal list	return mechanism.  What
       happens if you want to pass or return a hash?  Well, if you're using
       only one	of them, or you	don't mind them	concatenating, then the	normal
       calling convention is ok, although a little expensive.

       Where people get	into trouble is	here:

	   (@a,	@b) = func(@c, @d);
       or
	   (%a,	%b) = func(%c, %d);

       That syntax simply won't	work.  It sets just @a or %a and clears	the @b
       or %b.  Plus the	function didn't	get passed into	two separate arrays or
       hashes: it got one long list in @_, as always.

       If you can arrange for everyone to deal with this through references,
       it's cleaner code, although not so nice to look at.  Here's a function
       that takes two array references as arguments, returning the two array
       elements	in order of how	many elements they have	in them:

	   ($aref, $bref) = func(\@c, \@d);
	   print "@$aref has more than @$bref\n";
	   sub func {
	       my ($cref, $dref) = @_;
	       if (@$cref > @$dref) {
		   return ($cref, $dref);
	       } else {
		   return ($dref, $cref);
	       }
	   }

       It turns	out that you can actually do this also:

	   (*a,	*b) = func(\@c,	\@d);
	   print "@a has more than @b\n";
	   sub func {
	       local (*c, *d) =	@_;
	       if (@c >	@d) {
		   return (\@c,	\@d);
	       } else {
		   return (\@d,	\@c);
	       }
	   }

       Here we're using	the typeglobs to do symbol table aliasing.  It's a tad
       subtle, though, and also	won't work if you're using "my"	variables,
       because only globals (even in disguise as "local"s) are in the symbol
       table.

       If you're passing around	filehandles, you could usually just use	the
       bare typeglob, like *STDOUT, but	typeglobs references work, too.	 For
       example:

	   splutter(\*STDOUT);
	   sub splutter	{
	       my $fh =	shift;
	       print $fh "her um well a	hmmm\n";
	   }

	   $rec	= get_rec(\*STDIN);
	   sub get_rec {
	       my $fh =	shift;
	       return scalar <$fh>;
	   }

       If you're planning on generating	new filehandles, you could do this.
       Notice to pass back just	the bare *FH, not its reference.

	   sub openit {
	       my $path	= shift;
	       local *FH;
	       return open (FH,	$path) ? *FH : undef;
	   }

   Prototypes
       Perl supports a very limited kind of compile-time argument checking
       using function prototyping.  This can be	declared in either the PROTO
       section or with a prototype attribute.  If you declare either of

	   sub mypush (\@@)
	   sub mypush :prototype(\@@)

       then "mypush()" takes arguments exactly like "push()" does.

       If subroutine signatures	are enabled (see "Signatures"),	then the
       shorter PROTO syntax is unavailable, because it would clash with
       signatures.  In that case, a prototype can only be declared in the form
       of an attribute.

       The function declaration	must be	visible	at compile time.  The
       prototype affects only interpretation of	new-style calls	to the
       function, where new-style is defined as not using the "&" character.
       In other	words, if you call it like a built-in function,	then it
       behaves like a built-in function.  If you call it like an old-fashioned
       subroutine, then	it behaves like	an old-fashioned subroutine.  It
       naturally falls out from	this rule that prototypes have no influence on
       subroutine references like "\&foo" or on	indirect subroutine calls like
       "&{$subref}" or "$subref->()".

       Method calls are	not influenced by prototypes either, because the
       function	to be called is	indeterminate at compile time, since the exact
       code called depends on inheritance.

       Because the intent of this feature is primarily to let you define
       subroutines that	work like built-in functions, here are prototypes for
       some other functions that parse almost exactly like the corresponding
       built-in.

	  Declared as		  Called as

	  sub mylink ($$)	  mylink $old, $new
	  sub myvec ($$$)	  myvec	$var, $offset, 1
	  sub myindex ($$;$)	  myindex &getstring, "substr"
	  sub mysyswrite ($$$;$)  mysyswrite $buf, 0, length($buf) - $off, $off
	  sub myreverse	(@)	  myreverse $a,	$b, $c
	  sub myjoin ($@)	  myjoin ":", $a, $b, $c
	  sub mypop (\@)	  mypop	@array
	  sub mysplice (\@$$@)	  mysplice @array, 0, 2, @pushme
	  sub mykeys (\[%@])	  mykeys %{$hashref}
	  sub myopen (*;$)	  myopen HANDLE, $name
	  sub mypipe (**)	  mypipe READHANDLE, WRITEHANDLE
	  sub mygrep (&@)	  mygrep { /foo/ } $a, $b, $c
	  sub myrand (;$)	  myrand 42
	  sub mytime ()		  mytime

       Any backslashed prototype character represents an actual	argument that
       must start with that character (optionally preceded by "my", "our" or
       "local"), with the exception of "$", which will accept any scalar
       lvalue expression, such as "$foo	= 7" or	"my_function()->[0]".  The
       value passed as part of @_ will be a reference to the actual argument
       given in	the subroutine call, obtained by applying "\" to that
       argument.

       You can use the "\[]" backslash group notation to specify more than one
       allowed argument	type.  For example:

	   sub myref (\[$@%&*])

       will allow calling myref() as

	   myref $var
	   myref @array
	   myref %hash
	   myref &sub
	   myref *glob

       and the first argument of myref() will be a reference to	a scalar, an
       array, a	hash, a	code, or a glob.

       Unbackslashed prototype characters have special meanings.  Any
       unbackslashed "@" or "%"	eats all remaining arguments, and forces list
       context.	 An argument represented by "$"	forces scalar context.	An "&"
       requires	an anonymous subroutine, which,	if passed as the first
       argument, does not require the "sub" keyword or a subsequent comma.

       A "*" allows the	subroutine to accept a bareword, constant, scalar
       expression, typeglob, or	a reference to a typeglob in that slot.	 The
       value will be available to the subroutine either	as a simple scalar, or
       (in the latter two cases) as a reference	to the typeglob.  If you wish
       to always convert such arguments	to a typeglob reference, use
       Symbol::qualify_to_ref()	as follows:

	   use Symbol 'qualify_to_ref';

	   sub foo (*) {
	       my $fh =	qualify_to_ref(shift, caller);
	       ...
	   }

       The "+" prototype is a special alternative to "$" that will act like
       "\[@%]" when given a literal array or hash variable, but	will otherwise
       force scalar context on the argument.  This is useful for functions
       which should accept either a literal array or an	array reference	as the
       argument:

	   sub mypush (+@) {
	       my $aref	= shift;
	       die "Not	an array or arrayref" unless ref $aref eq 'ARRAY';
	       push @$aref, @_;
	   }

       When using the "+" prototype, your function must	check that the
       argument	is of an acceptable type.

       A semicolon (";") separates mandatory arguments from optional
       arguments.  It is redundant before "@" or "%", which gobble up
       everything else.

       As the last character of	a prototype, or	just before a semicolon, a "@"
       or a "%", you can use "_" in place of "$": if this argument is not
       provided, $_ will be used instead.

       Note how	the last three examples	in the table above are treated
       specially by the	parser.	 "mygrep()" is parsed as a true	list operator,
       "myrand()" is parsed as a true unary operator with unary	precedence the
       same as "rand()", and "mytime()"	is truly without arguments, just like
       "time()".  That is, if you say

	   mytime +2;

       you'll get "mytime() + 2", not mytime(2), which is how it would be
       parsed without a	prototype.  If you want	to force a unary function to
       have the	same precedence	as a list operator, add	";" to the end of the
       prototype:

	   sub mygetprotobynumber($;);
	   mygetprotobynumber $a > $b; # parsed	as mygetprotobynumber($a > $b)

       The interesting thing about "&" is that you can generate	new syntax
       with it,	provided it's in the initial position:

	   sub try (&@)	{
	       my($try,$catch) = @_;
	       eval { &$try };
	       if ($@) {
		   local $_ = $@;
		   &$catch;
	       }
	   }
	   sub catch (&) { $_[0] }

	   try {
	       die "phooey";
	   } catch {
	       /phooey/	and print "unphooey\n";
	   };

       That prints "unphooey".	(Yes, there are	still unresolved issues	having
       to do with visibility of	@_.  I'm ignoring that question	for the
       moment.	(But note that if we make @_ lexically scoped, those anonymous
       subroutines can act like	closures... (Gee, is this sounding a little
       Lispish?	 (Never	mind.))))

       And here's a reimplementation of	the Perl "grep"	operator:

	   sub mygrep (&@) {
	       my $code	= shift;
	       my @result;
	       foreach $_ (@_) {
		   push(@result, $_) if	&$code;
	       }
	       @result;
	   }

       Some folks would	prefer full alphanumeric prototypes.  Alphanumerics
       have been intentionally left out	of prototypes for the express purpose
       of someday in the future	adding named, formal parameters.  The current
       mechanism's main	goal is	to let module writers provide better
       diagnostics for module users.  Larry feels the notation quite
       understandable to Perl programmers, and that it will not	intrude
       greatly upon the	meat of	the module, nor	make it	harder to read.	 The
       line noise is visually encapsulated into	a small	pill that's easy to
       swallow.

       If you try to use an alphanumeric sequence in a prototype you will
       generate	an optional warning - "Illegal character in prototype...".
       Unfortunately earlier versions of Perl allowed the prototype to be used
       as long as its prefix was a valid prototype.  The warning may be
       upgraded	to a fatal error in a future version of	Perl once the majority
       of offending code is fixed.

       It's probably best to prototype new functions, not retrofit prototyping
       into older ones.	 That's	because	you must be especially careful about
       silent impositions of differing list versus scalar contexts.  For
       example,	if you decide that a function should take just one parameter,
       like this:

	   sub func ($)	{
	       my $n = shift;
	       print "you gave me $n\n";
	   }

       and someone has been calling it with an array or	expression returning a
       list:

	   func(@foo);
	   func( $text =~ /\w+/g );

       Then you've just	supplied an automatic "scalar" in front	of their
       argument, which can be more than	a bit surprising.  The old @foo	which
       used to hold one	thing doesn't get passed in.  Instead, "func()"	now
       gets passed in a	1; that	is, the	number of elements in @foo.  And the
       "m//g" gets called in scalar context so instead of a list of words it
       returns a boolean result	and advances "pos($text)".  Ouch!

       If a sub	has both a PROTO and a BLOCK, the prototype is not applied
       until after the BLOCK is	completely defined.  This means	that a
       recursive function with a prototype has to be predeclared for the
       prototype to take effect, like so:

	       sub foo($$);
	       sub foo($$) {
		       foo 1, 2;
	       }

       This is all very	powerful, of course, and should	be used	only in
       moderation to make the world a better place.

   Constant Functions
       Functions with a	prototype of "()" are potential	candidates for
       inlining.  If the result	after optimization and constant	folding	is
       either a	constant or a lexically-scoped scalar which has	no other
       references, then	it will	be used	in place of function calls made
       without "&".  Calls made	using "&" are never inlined.  (See constant.pm
       for an easy way to declare most constants.)

       The following functions would all be inlined:

	   sub pi ()	       { 3.14159 }	       # Not exact, but	close.
	   sub PI ()	       { 4 * atan2 1, 1	}      # As good as it gets,
						       # and it's inlined, too!
	   sub ST_DEV ()       { 0 }
	   sub ST_INO ()       { 1 }

	   sub FLAG_FOO	()     { 1 << 8	}
	   sub FLAG_BAR	()     { 1 << 9	}
	   sub FLAG_MASK ()    { FLAG_FOO | FLAG_BAR }

	   sub OPT_BAZ ()      { not (0x1B58 & FLAG_MASK) }

	   sub N () { int(OPT_BAZ) / 3 }

	   sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
	   sub FOO_SET2	() { if	(FLAG_MASK & FLAG_FOO) { 1 } }

       (Be aware that the last example was not always inlined in Perl 5.20 and
       earlier,	which did not behave consistently with subroutines containing
       inner scopes.)  You can countermand inlining by using an	explicit
       "return":

	   sub baz_val () {
	       if (OPT_BAZ) {
		   return 23;
	       }
	       else {
		   return 42;
	       }
	   }
	   sub bonk_val	() { return 12345 }

       As alluded to earlier you can also declare inlined subs dynamically at
       BEGIN time if their body	consists of a lexically-scoped scalar which
       has no other references.	 Only the first	example	here will be inlined:

	   BEGIN {
	       my $var = 1;
	       no strict 'refs';
	       *INLINED	= sub () { $var	};
	   }

	   BEGIN {
	       my $var = 1;
	       my $ref = \$var;
	       no strict 'refs';
	       *NOT_INLINED = sub () { $var };
	   }

       A not so	obvious	caveat with this (see [RT #79908]) is that the
       variable	will be	immediately inlined, and will stop behaving like a
       normal lexical variable,	e.g. this will print 79907, not	79908:

	   BEGIN {
	       my $x = 79907;
	       *RT_79908 = sub () { $x };
	       $x++;
	   }
	   print RT_79908(); # prints 79907

       As of Perl 5.22,	this buggy behavior, while preserved for backward
       compatibility, is detected and emits a deprecation warning.  If you
       want the	subroutine to be inlined (with no warning), make sure the
       variable	is not used in a context where it could	be modified aside from
       where it	is declared.

	   # Fine, no warning
	   BEGIN {
	       my $x = 54321;
	       *INLINED	= sub () { $x };
	   }
	   # Warns.  Future Perl versions will stop inlining it.
	   BEGIN {
	       my $x;
	       $x = 54321;
	       *ALSO_INLINED = sub () {	$x };
	   }

       Perl 5.22 also introduces the experimental "const" attribute as an
       alternative.  (Disable the "experimental::const_attr" warnings if you
       want to use it.)	 When applied to an anonymous subroutine, it forces
       the sub to be called when the "sub" expression is evaluated.  The
       return value is captured	and turned into	a constant subroutine:

	   my $x = 54321;
	   *INLINED = sub : const { $x };
	   $x++;

       The return value	of "INLINED" in	this example will always be 54321,
       regardless of later modifications to $x.	 You can also put any
       arbitrary code inside the sub, at it will be executed immediately and
       its return value	captured the same way.

       If you really want a subroutine with a "()" prototype that returns a
       lexical variable	you can	easily force it	to not be inlined by adding an
       explicit	"return":

	   BEGIN {
	       my $x = 79907;
	       *RT_79908 = sub () { return $x };
	       $x++;
	   }
	   print RT_79908(); # prints 79908

       The easiest way to tell if a subroutine was inlined is by using
       B::Deparse.  Consider this example of two subroutines returning 1, one
       with a "()" prototype causing it	to be inlined, and one without (with
       deparse output truncated	for clarity):

	$ perl -MO=Deparse -le 'sub ONE	{ 1 } if (ONE) { print ONE if ONE }'
	sub ONE	{
	    1;
	}
	if (ONE	) {
	    print ONE()	if ONE ;
	}
	$ perl -MO=Deparse -le 'sub ONE	() { 1 } if (ONE) { print ONE if ONE }'
	sub ONE	() { 1 }
	do {
	    print 1
	};

       If you redefine a subroutine that was eligible for inlining, you'll get
       a warning by default.  You can use this warning to tell whether or not
       a particular subroutine is considered inlinable,	since it's different
       than the	warning	for overriding non-inlined subroutines:

	   $ perl -e 'sub one () {1} sub one ()	{2}'
	   Constant subroutine one redefined at	-e line	1.
	   $ perl -we 'sub one {1} sub one {2}'
	   Subroutine one redefined at -e line 1.

       The warning is considered severe	enough not to be affected by the -w
       switch (or its absence) because previously compiled invocations of the
       function	will still be using the	old value of the function.  If you
       need to be able to redefine the subroutine, you need to ensure that it
       isn't inlined, either by	dropping the "()" prototype (which changes
       calling semantics, so beware) or	by thwarting the inlining mechanism in
       some other way, e.g. by adding an explicit "return", as mentioned
       above:

	   sub not_inlined () {	return 23 }

   Overriding Built-in Functions
       Many built-in functions may be overridden, though this should be	tried
       only occasionally and for good reason.  Typically this might be done by
       a package attempting to emulate missing built-in	functionality on a
       non-Unix	system.

       Overriding may be done only by importing	the name from a	module at
       compile time--ordinary predeclaration isn't good	enough.	 However, the
       "use subs" pragma lets you, in effect, predeclare subs via the import
       syntax, and these names may then	override built-in ones:

	   use subs 'chdir', 'chroot', 'chmod',	'chown';
	   chdir $somewhere;
	   sub chdir { ... }

       To unambiguously	refer to the built-in form, precede the	built-in name
       with the	special	package	qualifier "CORE::".  For example, saying
       "CORE::open()" always refers to the built-in "open()", even if the
       current package has imported some other subroutine called "&open()"
       from elsewhere.	Even though it looks like a regular function call, it
       isn't: the CORE:: prefix	in that	case is	part of	Perl's syntax, and
       works for any keyword, regardless of what is in the CORE	package.
       Taking a	reference to it, that is, "\&CORE::open", only works for some
       keywords.  See CORE.

       Library modules should not in general export built-in names like	"open"
       or "chdir" as part of their default @EXPORT list, because these may
       sneak into someone else's namespace and change the semantics
       unexpectedly.  Instead, if the module adds that name to @EXPORT_OK,
       then it's possible for a	user to	import the name	explicitly, but	not
       implicitly.  That is, they could	say

	   use Module 'open';

       and it would import the "open" override.	 But if	they said

	   use Module;

       they would get the default imports without overrides.

       The foregoing mechanism for overriding built-in is restricted, quite
       deliberately, to	the package that requests the import.  There is	a
       second method that is sometimes applicable when you wish	to override a
       built-in	everywhere, without regard to namespace	boundaries.  This is
       achieved	by importing a sub into	the special namespace
       "CORE::GLOBAL::".  Here is an example that quite	brazenly replaces the
       "glob" operator with something that understands regular expressions.

	   package REGlob;
	   require Exporter;
	   @ISA	= 'Exporter';
	   @EXPORT_OK =	'glob';

	   sub import {
	       my $pkg = shift;
	       return unless @_;
	       my $sym = shift;
	       my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
	       $pkg->export($where, $sym, @_);
	   }

	   sub glob {
	       my $pat = shift;
	       my @got;
	       if (opendir my $d, '.') {
		   @got	= grep /$pat/, readdir $d;
		   closedir $d;
	       }
	       return @got;
	   }
	   1;

       And here's how it could be (ab)used:

	   #use	REGlob 'GLOBAL_glob';	   # override glob() in	ALL namespaces
	   package Foo;
	   use REGlob 'glob';		   # override glob() in	Foo:: only
	   print for <^[a-z_]+\.pm\$>;	   # show all pragmatic	modules

       The initial comment shows a contrived, even dangerous example.  By
       overriding "glob" globally, you would be	forcing	the new	(and
       subversive) behavior for	the "glob" operator for	every namespace,
       without the complete cognizance or cooperation of the modules that own
       those namespaces.  Naturally, this should be done with extreme
       caution--if it must be done at all.

       The "REGlob" example above does not implement all the support needed to
       cleanly override	perl's "glob" operator.	 The built-in "glob" has
       different behaviors depending on	whether	it appears in a	scalar or list
       context,	but our	"REGlob" doesn't.  Indeed, many	perl built-in have
       such context sensitive behaviors, and these must	be adequately
       supported by a properly written override.  For a	fully functional
       example of overriding "glob", study the implementation of
       "File::DosGlob" in the standard library.

       When you	override a built-in, your replacement should be	consistent (if
       possible) with the built-in native syntax.  You can achieve this	by
       using a suitable	prototype.  To get the prototype of an overridable
       built-in, use the "prototype" function with an argument of
       "CORE::builtin_name" (see "prototype" in	perlfunc).

       Note however that some built-ins	can't have their syntax	expressed by a
       prototype (such as "system" or "chomp").	 If you	override them you
       won't be	able to	fully mimic their original syntax.

       The built-ins "do", "require" and "glob"	can also be overridden,	but
       due to special magic, their original syntax is preserved, and you don't
       have to define a	prototype for their replacements.  (You	can't override
       the "do BLOCK" syntax, though).

       "require" has special additional	dark magic: if you invoke your
       "require" replacement as	"require Foo::Bar", it will actually receive
       the argument "Foo/Bar.pm" in @_.	 See "require" in perlfunc.

       And, as you'll have noticed from	the previous example, if you override
       "glob", the "<*>" glob operator is overridden as	well.

       In a similar fashion, overriding	the "readline" function	also overrides
       the equivalent I/O operator "<FILEHANDLE>".  Also, overriding
       "readpipe" also overrides the operators "``" and	"qx//".

       Finally,	some built-ins (e.g. "exists" or "grep") can't be overridden.

   Autoloading
       If you call a subroutine	that is	undefined, you would ordinarily	get an
       immediate, fatal	error complaining that the subroutine doesn't exist.
       (Likewise for subroutines being used as methods,	when the method
       doesn't exist in	any base class of the class's package.)	 However, if
       an "AUTOLOAD" subroutine	is defined in the package or packages used to
       locate the original subroutine, then that "AUTOLOAD" subroutine is
       called with the arguments that would have been passed to	the original
       subroutine.  The	fully qualified	name of	the original subroutine
       magically appears in the	global $AUTOLOAD variable of the same package
       as the "AUTOLOAD" routine.  The name is not passed as an	ordinary
       argument	because, er, well, just	because, that's	why.  (As an
       exception, a method call	to a nonexistent "import" or "unimport"	method
       is just skipped instead.	 Also, if the AUTOLOAD subroutine is an	XSUB,
       there are other ways to retrieve	the subroutine name.  See "Autoloading
       with XSUBs" in perlguts for details.)

       Many "AUTOLOAD" routines	load in	a definition for the requested
       subroutine using	eval(),	then execute that subroutine using a special
       form of goto() that erases the stack frame of the "AUTOLOAD" routine
       without a trace.	 (See the source to the	standard module	documented in
       AutoLoader, for example.)  But an "AUTOLOAD" routine can	also just
       emulate the routine and never define it.	  For example, let's pretend
       that a function that wasn't defined should just invoke "system" with
       those arguments.	 All you'd do is:

	   sub AUTOLOAD	{
	       my $program = $AUTOLOAD;
	       $program	=~ s/.*:://;
	       system($program,	@_);
	   }
	   date();
	   who('am', 'i');
	   ls('-l');

       In fact,	if you predeclare functions you	want to	call that way, you
       don't even need parentheses:

	   use subs qw(date who	ls);
	   date;
	   who "am", "i";
	   ls '-l';

       A more complete example of this is the Shell module on CPAN, which can
       treat undefined subroutine calls	as calls to external programs.

       Mechanisms are available	to help	modules	writers	split their modules
       into autoloadable files.	 See the standard AutoLoader module described
       in AutoLoader and in AutoSplit, the standard SelfLoader modules in
       SelfLoader, and the document on adding C	functions to Perl code in
       perlxs.

   Subroutine Attributes
       A subroutine declaration	or definition may have a list of attributes
       associated with it.  If such an attribute list is present, it is	broken
       up at space or colon boundaries and treated as though a "use
       attributes" had been seen.  See attributes for details about what
       attributes are currently	supported.  Unlike the limitation with the
       obsolescent "use	attrs",	the "sub : ATTRLIST" syntax works to associate
       the attributes with a pre-declaration, and not just with	a subroutine
       definition.

       The attributes must be valid as simple identifier names (without	any
       punctuation other than the '_' character).  They	may have a parameter
       list appended, which is only checked for	whether	its parentheses
       ('(',')') nest properly.

       Examples	of valid syntax	(even though the attributes are	unknown):

	   sub fnord (&\%) : switch(10,foo(7,3))  :  expensive;
	   sub plugh ()	: Ugly('\(") :Bad;
	   sub xyzzy : _5x5 { ... }

       Examples	of invalid syntax:

	   sub fnord : switch(10,foo();	# ()-string not	balanced
	   sub snoid : Ugly('(');	 # ()-string not balanced
	   sub xyzzy : 5x5;		 # "5x5" not a valid identifier
	   sub plugh : Y2::north;	 # "Y2::north" not a simple identifier
	   sub snurt : foo + bar;	 # "+" not a colon or space

       The attribute list is passed as a list of constant strings to the code
       which associates	them with the subroutine.  In particular, the second
       example of valid	syntax above currently looks like this in terms	of how
       it's parsed and invoked:

	   use attributes __PACKAGE__, \&plugh,	q[Ugly('\(")], 'Bad';

       For further details on attribute	lists and their	manipulation, see
       attributes and Attribute::Handlers.

SEE ALSO
       See "Function Templates"	in perlref for more about references and
       closures.  See perlxs if	you'd like to learn about calling C
       subroutines from	Perl.  See perlembed if	you'd like to learn about
       calling Perl subroutines	from C.	 See perlmod to	learn about bundling
       up your functions in separate files.  See perlmodlib to learn what
       library modules come standard on	your system.  See perlootut to learn
       how to make object method calls.

perl v5.28.3			  2020-05-14			    PERLSUB(1)

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO

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