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PERLSYN(1)	       Perl Programmers	Reference Guide		    PERLSYN(1)

NAME
       perlsyn - Perl syntax

DESCRIPTION
       A Perl program consists of a sequence of	declarations and statements
       which run from the top to the bottom.  Loops, subroutines, and other
       control structures allow	you to jump around within the code.

       Perl is a free-form language: you can format and	indent it however you
       like.  Whitespace serves	mostly to separate tokens, unlike languages
       like Python where it is an important part of the	syntax,	or Fortran
       where it	is immaterial.

       Many of Perl's syntactic	elements are optional.	Rather than requiring
       you to put parentheses around every function call and declare every
       variable, you can often leave such explicit elements off	and Perl will
       figure out what you meant.  This	is known as Do What I Mean,
       abbreviated DWIM.  It allows programmers	to be lazy and to code in a
       style with which	they are comfortable.

       Perl borrows syntax and concepts	from many languages: awk, sed, C,
       Bourne Shell, Smalltalk,	Lisp and even English.	Other languages	have
       borrowed	syntax from Perl, particularly its regular expression
       extensions.  So if you have programmed in another language you will see
       familiar	pieces in Perl.	 They often work the same, but see perltrap
       for information about how they differ.

   Declarations
       The only	things you need	to declare in Perl are report formats and
       subroutines (and	sometimes not even subroutines).  A scalar variable
       holds the undefined value ("undef") until it has	been assigned a
       defined value, which is anything	other than "undef".  When used as a
       number, "undef" is treated as 0;	when used as a string, it is treated
       as the empty string, "";	and when used as a reference that isn't	being
       assigned	to, it is treated as an	error.	If you enable warnings,	you'll
       be notified of an uninitialized value whenever you treat	"undef"	as a
       string or a number.  Well, usually.  Boolean contexts, such as:

	   if ($a) {}

       are exempt from warnings	(because they care about truth rather than
       definedness).  Operators	such as	"++", "--", "+=", "-=",	and ".=", that
       operate on undefined variables such as:

	   undef $a;
	   $a++;

       are also	always exempt from such	warnings.

       A declaration can be put	anywhere a statement can, but has no effect on
       the execution of	the primary sequence of	statements: declarations all
       take effect at compile time.  All declarations are typically put	at the
       beginning or the	end of the script.  However, if	you're using
       lexically-scoped	private	variables created with "my()", "state()", or
       "our()",	you'll have to make sure your format or	subroutine definition
       is within the same block	scope as the my	if you expect to be able to
       access those private variables.

       Declaring a subroutine allows a subroutine name to be used as if	it
       were a list operator from that point forward in the program.  You can
       declare a subroutine without defining it	by saying "sub name", thus:

	   sub myname;
	   $me = myname	$0	       or die "can't get myname";

       A bare declaration like that declares the function to be	a list
       operator, not a unary operator, so you have to be careful to use
       parentheses (or "or" instead of "||".)  The "||"	operator binds too
       tightly to use after list operators; it becomes part of the last
       element.	 You can always	use parentheses	around the list	operators
       arguments to turn the list operator back	into something that behaves
       more like a function call.  Alternatively, you can use the prototype
       "($)" to	turn the subroutine into a unary operator:

	 sub myname ($);
	 $me = myname $0	     ||	die "can't get myname";

       That now	parses as you'd	expect,	but you	still ought to get in the
       habit of	using parentheses in that situation.  For more on prototypes,
       see perlsub.

       Subroutines declarations	can also be loaded up with the "require"
       statement or both loaded	and imported into your namespace with a	"use"
       statement.  See perlmod for details on this.

       A statement sequence may	contain	declarations of	lexically-scoped
       variables, but apart from declaring a variable name, the	declaration
       acts like an ordinary statement,	and is elaborated within the sequence
       of statements as	if it were an ordinary statement.  That	means it
       actually	has both compile-time and run-time effects.

   Comments
       Text from a "#" character until the end of the line is a	comment, and
       is ignored.  Exceptions include "#" inside a string or regular
       expression.

   Simple Statements
       The only	kind of	simple statement is an expression evaluated for	its
       side-effects.  Every simple statement must be terminated	with a
       semicolon, unless it is the final statement in a	block, in which	case
       the semicolon is	optional.  But put the semicolon in anyway if the
       block takes up more than	one line, because you may eventually add
       another line.  Note that	there are operators like "eval {}", "sub {}",
       and "do {}" that	look like compound statements, but aren't--they're
       just TERMs in an	expression--and	thus need an explicit termination when
       used as the last	item in	a statement.

   Statement Modifiers
       Any simple statement may	optionally be followed by a SINGLE modifier,
       just before the terminating semicolon (or block ending).	 The possible
       modifiers are:

	   if EXPR
	   unless EXPR
	   while EXPR
	   until EXPR
	   for LIST
	   foreach LIST
	   when	EXPR

       The "EXPR" following the	modifier is referred to	as the "condition".
       Its truth or falsehood determines how the modifier will behave.

       "if" executes the statement once	if and only if the condition is	true.
       "unless"	is the opposite, it executes the statement unless the
       condition is true (that is, if the condition is false).	See "Scalar
       values" in perldata for definitions of true and false.

	   print "Basset hounds	got long ears" if length $ear >= 10;
	   go_outside()	and play() unless $is_raining;

       The "for(each)" modifier	is an iterator:	it executes the	statement once
       for each	item in	the LIST (with $_ aliased to each item in turn).
       There is	no syntax to specify a C-style for loop	or a lexically scoped
       iteration variable in this form.

	   print "Hello	$_!\n" for qw(world Dolly nurse);

       "while" repeats the statement while the condition is true.  Postfix
       "while" has the same magic treatment of some kinds of condition that
       prefix "while" has.  "until" does the opposite, it repeats the
       statement until the condition is	true (or while the condition is
       false):

	   # Both of these count from 0	to 10.
	   print $i++ while $i <= 10;
	   print $j++ until $j >  10;

       The "while" and "until" modifiers have the usual	""while" loop"
       semantics (conditional evaluated	first),	except when applied to a
       "do"-BLOCK (or to the Perl4 "do"-SUBROUTINE statement), in which	case
       the block executes once before the conditional is evaluated.

       This is so that you can write loops like:

	   do {
	       $line = <STDIN>;
	       ...
	   } until !defined($line) || $line eq ".\n"

       See "do"	in perlfunc.  Note also	that the loop control statements
       described later will NOT	work in	this construct,	because	modifiers
       don't take loop labels.	Sorry.	You can	always put another block
       inside of it (for "next"/"redo")	or around it (for "last") to do	that
       sort of thing.

       For "next" or "redo", just double the braces:

	   do {{
	       next if $x == $y;
	       # do something here
	   }} until $x++ > $z;

       For "last", you have to be more elaborate and put braces	around it:

	   {
	       do {
		   last	if $x == $y**2;
		   # do	something here
	       } while $x++ <= $z;
	   }

       If you need both	"next" and "last", you have to do both and also	use a
       loop label:

	   LOOP: {
	       do {{
		   next	if $x == $y;
		   last	LOOP if	$x == $y**2;
		   # do	something here
	       }} until	$x++ > $z;
	   }

       NOTE: The behaviour of a	"my", "state", or "our"	modified with a
       statement modifier conditional or loop construct	(for example, "my $x
       if ...")	is undefined.  The value of the	"my" variable may be "undef",
       any previously assigned value, or possibly anything else.  Don't	rely
       on it.  Future versions of perl might do	something different from the
       version of perl you try it out on.  Here	be dragons.

       The "when" modifier is an experimental feature that first appeared in
       Perl 5.14.  To use it, you should include a "use	v5.14" declaration.
       (Technically, it	requires only the "switch" feature, but	that aspect of
       it was not available before 5.14.)  Operative only from within a
       "foreach" loop or a "given" block, it executes the statement only if
       the smartmatch "$_ ~~ EXPR" is true.  If	the statement executes,	it is
       followed	by a "next" from inside	a "foreach" and	"break"	from inside a
       "given".

       Under the current implementation, the "foreach" loop can	be anywhere
       within the "when" modifier's dynamic scope, but must be within the
       "given" block's lexical scope.  This restriction	may be relaxed in a
       future release.	See "Switch Statements"	below.

   Compound Statements
       In Perl,	a sequence of statements that defines a	scope is called	a
       block.  Sometimes a block is delimited by the file containing it	(in
       the case	of a required file, or the program as a	whole),	and sometimes
       a block is delimited by the extent of a string (in the case of an
       eval).

       But generally, a	block is delimited by curly brackets, also known as
       braces.	We will	call this syntactic construct a	BLOCK.	Because
       enclosing braces	are also the syntax for	hash reference constructor
       expressions (see	perlref), you may occasionally need to disambiguate by
       placing a ";" immediately after an opening brace	so that	Perl realises
       the brace is the	start of a block.  You will more frequently need to
       disambiguate the	other way, by placing a	"+" immediately	before an
       opening brace to	force it to be interpreted as a	hash reference
       constructor expression.	It is considered good style to use these
       disambiguating mechanisms liberally, not	only when Perl would otherwise
       guess incorrectly.

       The following compound statements may be	used to	control	flow:

	   if (EXPR) BLOCK
	   if (EXPR) BLOCK else	BLOCK
	   if (EXPR) BLOCK elsif (EXPR)	BLOCK ...
	   if (EXPR) BLOCK elsif (EXPR)	BLOCK ... else BLOCK

	   unless (EXPR) BLOCK
	   unless (EXPR) BLOCK else BLOCK
	   unless (EXPR) BLOCK elsif (EXPR) BLOCK ...
	   unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK

	   given (EXPR)	BLOCK

	   LABEL while (EXPR) BLOCK
	   LABEL while (EXPR) BLOCK continue BLOCK

	   LABEL until (EXPR) BLOCK
	   LABEL until (EXPR) BLOCK continue BLOCK

	   LABEL for (EXPR; EXPR; EXPR)	BLOCK
	   LABEL for VAR (LIST)	BLOCK
	   LABEL for VAR (LIST)	BLOCK continue BLOCK

	   LABEL foreach (EXPR;	EXPR; EXPR) BLOCK
	   LABEL foreach VAR (LIST) BLOCK
	   LABEL foreach VAR (LIST) BLOCK continue BLOCK

	   LABEL BLOCK
	   LABEL BLOCK continue	BLOCK

	   PHASE BLOCK

       If enabled by the experimental "try" feature, the following may also be
       used

	   try BLOCK catch (VAR) BLOCK

       The experimental	"given"	statement is not automatically enabled;	see
       "Switch Statements" below for how to do so, and the attendant caveats.

       Unlike in C and Pascal, in Perl these are all defined in	terms of
       BLOCKs, not statements.	This means that	the curly brackets are
       required--no dangling statements	allowed.  If you want to write
       conditionals without curly brackets, there are several other ways to do
       it.  The	following all do the same thing:

	   if (!open(FOO)) { die "Can't	open $FOO: $!" }
	   die "Can't open $FOO: $!" unless open(FOO);
	   open(FOO)  || die "Can't open $FOO: $!";
	   open(FOO) ? () : die	"Can't open $FOO: $!";
	       # a bit exotic, that last one

       The "if"	statement is straightforward.  Because BLOCKs are always
       bounded by curly	brackets, there	is never any ambiguity about which
       "if" an "else" goes with.  If you use "unless" in place of "if",	the
       sense of	the test is reversed.  Like "if", "unless" can be followed by
       "else".	"unless" can even be followed by one or	more "elsif"
       statements, though you may want to think	twice before using that
       particular language construct, as everyone reading your code will have
       to think	at least twice before they can understand what's going on.

       The "while" statement executes the block	as long	as the expression is
       true.  The "until" statement executes the block as long as the
       expression is false.  The LABEL is optional, and	if present, consists
       of an identifier	followed by a colon.  The LABEL	identifies the loop
       for the loop control statements "next", "last", and "redo".  If the
       LABEL is	omitted, the loop control statement refers to the innermost
       enclosing loop.	This may include dynamically searching through your
       call-stack at run time to find the LABEL.  Such desperate behavior
       triggers	a warning if you use the "use warnings"	pragma or the -w flag.

       If the condition	expression of a	"while"	statement is based on any of a
       group of	iterative expression types then	it gets	some magic treatment.
       The affected iterative expression types are "readline", the
       "<FILEHANDLE>" input operator, "readdir", "glob", the "<PATTERN>"
       globbing	operator, and "each".  If the condition	expression is one of
       these expression	types, then the	value yielded by the iterative
       operator	will be	implicitly assigned to $_.  If the condition
       expression is one of these expression types or an explicit assignment
       of one of them to a scalar, then	the condition actually tests for
       definedness of the expression's value, not for its regular truth	value.

       If there	is a "continue"	BLOCK, it is always executed just before the
       conditional is about to be evaluated again.  Thus it can	be used	to
       increment a loop	variable, even when the	loop has been continued	via
       the "next" statement.

       When a block is preceded	by a compilation phase keyword such as
       "BEGIN",	"END", "INIT", "CHECK",	or "UNITCHECK",	then the block will
       run only	during the corresponding phase of execution.  See perlmod for
       more details.

       Extension modules can also hook into the	Perl parser to define new
       kinds of	compound statements.  These are	introduced by a	keyword	which
       the extension recognizes, and the syntax	following the keyword is
       defined entirely	by the extension.  If you are an implementor, see
       "PL_keyword_plugin" in perlapi for the mechanism.  If you are using
       such a module, see the module's documentation for details of the	syntax
       that it defines.

   Loop	Control
       The "next" command starts the next iteration of the loop:

	   LINE: while (<STDIN>) {
	       next LINE if /^#/;      # discard comments
	       ...
	   }

       The "last" command immediately exits the	loop in	question.  The
       "continue" block, if any, is not	executed:

	   LINE: while (<STDIN>) {
	       last LINE if /^$/;      # exit when done	with header
	       ...
	   }

       The "redo" command restarts the loop block without evaluating the
       conditional again.  The "continue" block, if any, is not	executed.
       This command is normally	used by	programs that want to lie to
       themselves about	what was just input.

       For example, when processing a file like	/etc/termcap.  If your input
       lines might end in backslashes to indicate continuation,	you want to
       skip ahead and get the next record.

	   while (<>) {
	       chomp;
	       if (s/\\$//) {
		   $_ .= <>;
		   redo	unless eof();
	       }
	       # now process $_
	   }

       which is	Perl shorthand for the more explicitly written version:

	   LINE: while (defined($line =	<ARGV>)) {
	       chomp($line);
	       if ($line =~ s/\\$//) {
		   $line .= <ARGV>;
		   redo	LINE unless eof(); # not eof(ARGV)!
	       }
	       # now process $line
	   }

       Note that if there were a "continue" block on the above code, it	would
       get executed only on lines discarded by the regex (since	redo skips the
       continue	block).	 A continue block is often used	to reset line counters
       or "m?pat?" one-time matches:

	   # inspired by :1,$g/fred/s//WILMA/
	   while (<>) {
	       m?(fred)?    && s//WILMA	$1 WILMA/;
	       m?(barney)?  && s//BETTY	$1 BETTY/;
	       m?(homer)?   && s//MARGE	$1 MARGE/;
	   } continue {
	       print "$ARGV $.:	$_";
	       close ARGV  if eof;	       # reset $.
	       reset	   if eof;	       # reset ?pat?
	   }

       If the word "while" is replaced by the word "until", the	sense of the
       test is reversed, but the conditional is	still tested before the	first
       iteration.

       Loop control statements don't work in an	"if" or	"unless", since	they
       aren't loops.  You can double the braces	to make	them such, though.

	   if (/pattern/) {{
	       last if /fred/;
	       next if /barney/; # same	effect as "last",
				 # but doesn't document	as well
	       # do something here
	   }}

       This is caused by the fact that a block by itself acts as a loop	that
       executes	once, see "Basic BLOCKs".

       The form	"while/if BLOCK	BLOCK",	available in Perl 4, is	no longer
       available.   Replace any	occurrence of "if BLOCK" by "if	(do BLOCK)".

   For Loops
       Perl's C-style "for" loop works like the	corresponding "while" loop;
       that means that this:

	   for ($i = 1;	$i < 10; $i++) {
	       ...
	   }

       is the same as this:

	   $i =	1;
	   while ($i < 10) {
	       ...
	   } continue {
	       $i++;
	   }

       There is	one minor difference: if variables are declared	with "my" in
       the initialization section of the "for",	the lexical scope of those
       variables is exactly the	"for" loop (the	body of	the loop and the
       control sections).  To illustrate:

	   my $i = 'samba';
	   for (my $i =	1; $i <= 4; $i++) {
	       print "$i\n";
	   }
	   print "$i\n";

       when executed, gives:

	   1
	   2
	   3
	   4
	   samba

       As a special case, if the test in the "for" loop	(or the	corresponding
       "while" loop) is	empty, it is treated as	true.  That is,	both

	   for (;;) {
	       ...
	   }

       and

	   while () {
	       ...
	   }

       are treated as infinite loops.

       Besides the normal array	index looping, "for" can lend itself to	many
       other interesting applications.	Here's one that	avoids the problem you
       get into	if you explicitly test for end-of-file on an interactive file
       descriptor causing your program to appear to hang.

	   $on_a_tty = -t STDIN	&& -t STDOUT;
	   sub prompt {	print "yes? " if $on_a_tty }
	   for ( prompt(); <STDIN>; prompt() ) {
	       # do something
	   }

       The condition expression	of a "for" loop	gets the same magic treatment
       of "readline" et	al that	the condition expression of a "while" loop
       gets.

   Foreach Loops
       The "foreach" loop iterates over	a normal list value and	sets the
       scalar variable VAR to be each element of the list in turn.  If the
       variable	is preceded with the keyword "my", then	it is lexically
       scoped, and is therefore	visible	only within the	loop.  Otherwise, the
       variable	is implicitly local to the loop	and regains its	former value
       upon exiting the	loop.  If the variable was previously declared with
       "my", it	uses that variable instead of the global one, but it's still
       localized to the	loop.  This implicit localization occurs only in a
       "foreach" loop.

       The "foreach" keyword is	actually a synonym for the "for" keyword, so
       you can use either.  If VAR is omitted, $_ is set to each value.

       If any element of LIST is an lvalue, you	can modify it by modifying VAR
       inside the loop.	 Conversely, if	any element of LIST is NOT an lvalue,
       any attempt to modify that element will fail.  In other words, the
       "foreach" loop index variable is	an implicit alias for each item	in the
       list that you're	looping	over.

       If any part of LIST is an array,	"foreach" will get very	confused if
       you add or remove elements within the loop body,	for example with
       "splice".   So don't do that.

       "foreach" probably won't	do what	you expect if VAR is a tied or other
       special variable.   Don't do that either.

       As of Perl 5.22,	there is an experimental variant of this loop that
       accepts a variable preceded by a	backslash for VAR, in which case the
       items in	the LIST must be references.  The backslashed variable will
       become an alias to each referenced item in the LIST, which must be of
       the correct type.  The variable needn't be a scalar in this case, and
       the backslash may be followed by	"my".  To use this form, you must
       enable the "refaliasing"	feature	via "use feature".  (See feature.  See
       also "Assigning to References" in perlref.)

       Examples:

	   for (@ary) {	s/foo/bar/ }

	   for my $elem	(@elements) {
	       $elem *=	2;
	   }

	   for $count (reverse(1..10), "BOOM") {
	       print $count, "\n";
	       sleep(1);
	   }

	   for (1..15) { print "Merry Christmas\n"; }

	   foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
	       print "Item: $item\n";
	   }

	   use feature "refaliasing";
	   no warnings "experimental::refaliasing";
	   foreach \my %hash (@array_of_hash_references) {
	       # do something which each %hash
	   }

       Here's how a C programmer might code up a particular algorithm in Perl:

	   for (my $i =	0; $i <	@ary1; $i++) {
	       for (my $j = 0; $j < @ary2; $j++) {
		   if ($ary1[$i] > $ary2[$j]) {
		       last; # can't go	to outer :-(
		   }
		   $ary1[$i] +=	$ary2[$j];
	       }
	       # this is where that last takes me
	   }

       Whereas here's how a Perl programmer more comfortable with the idiom
       might do	it:

	   OUTER: for my $wid (@ary1) {
	   INNER:   for	my $jet	(@ary2)	{
		       next OUTER if $wid > $jet;
		       $wid += $jet;
		    }
		 }

       See how much easier this	is?  It's cleaner, safer, and faster.  It's
       cleaner because it's less noisy.	 It's safer because if code gets added
       between the inner and outer loops later on, the new code	won't be
       accidentally executed.  The "next" explicitly iterates the other	loop
       rather than merely terminating the inner	one.  And it's faster because
       Perl executes a "foreach" statement more	rapidly	than it	would the
       equivalent C-style "for"	loop.

       Perceptive Perl hackers may have	noticed	that a "for" loop has a	return
       value, and that this value can be captured by wrapping the loop in a
       "do" block.  The	reward for this	discovery is this cautionary advice:
       The return value	of a "for" loop	is unspecified and may change without
       notice.	Do not rely on it.

   Try Catch Exception Handling
       The "try"/"catch" syntax	provides control flow relating to exception
       handling. The "try" keyword introduces a	block which will be executed
       when it is encountered, and the "catch" block provides code to handle
       any exception that may be thrown	by the first.

	   try {
	       my $x = call_a_function();
	       $x < 100	or die "Too big";
	       send_output($x);
	   }
	   catch ($e) {
	       warn "Unable to output a	value; $e";
	   }
	   print "Finished\n";

       Here, the body of the "catch" block (i.e. the "warn" statement) will be
       executed	if the initial block invokes the conditional "die", or if
       either of the functions it invokes throws an uncaught exception.	The
       "catch" block can inspect the $e	lexical	variable in this case to see
       what the	exception was.	If no exception	was thrown then	the "catch"
       block does not happen. In either	case, execution	will then continue
       from the	following statement - in this example the "print".

       The "catch" keyword must	be immediately followed	by a variable
       declaration in parentheses, which introduces a new variable visible to
       the body	of the subsequent block. Inside	the block this variable	will
       contain the exception value that	was thrown by the code in the "try"
       block. It is not	necessary to use the "my" keyword to declare this
       variable; this is implied (similar as it	is for subroutine signatures).

       Both the	"try" and the "catch" blocks are permitted to contain control-
       flow expressions, such as "return", "goto", or "next"/"last"/"redo". In
       all cases they behave as	expected without warnings. In particular, a
       "return"	expression inside the "try" block will make its	entire
       containing function return - this is in contrast	to its behaviour
       inside an "eval"	block, where it	would only make	that block return.

       Like other control-flow syntax, "try" and "catch" will yield the	last
       evaluated value when placed as the final	statement in a function	or a
       "do" block. This	permits	the syntax to be used to create	a value. In
       this case remember not to use the "return" expression, or that will
       cause the containing function to	return.

	   my $value = do {
	       try {
		   get_thing(@args);
	       }
	       catch ($e) {
		   warn	"Unable	to get thing - $e";
		   $DEFAULT_THING;
	       }
	   };

       As with other control-flow syntax, "try"	blocks are not visible to
       "caller()" (just	as for example,	"while"	or "foreach" loops are not).
       Successive levels of the	"caller" result	can see	subroutine calls and
       "eval" blocks, because those affect the way that	"return" would work.
       Since "try" blocks do not intercept "return", they are not of interest
       to "caller".

       This syntax is currently	experimental and must be enabled with "use
       feature 'try'". It emits	a warning in the "experimental::try" category.

   Basic BLOCKs
       A BLOCK by itself (labeled or not) is semantically equivalent to	a loop
       that executes once.  Thus you can use any of the	loop control
       statements in it	to leave or restart the	block.	(Note that this	is NOT
       true in "eval{}", "sub{}", or contrary to popular belief	"do{}" blocks,
       which do	NOT count as loops.)  The "continue" block is optional.

       The BLOCK construct can be used to emulate case structures.

	   SWITCH: {
	       if (/^abc/) { $abc = 1; last SWITCH; }
	       if (/^def/) { $def = 1; last SWITCH; }
	       if (/^xyz/) { $xyz = 1; last SWITCH; }
	       $nothing	= 1;
	   }

       You'll also find	that "foreach" loop used to create a topicalizer and a
       switch:

	   SWITCH:
	   for ($var) {
	       if (/^abc/) { $abc = 1; last SWITCH; }
	       if (/^def/) { $def = 1; last SWITCH; }
	       if (/^xyz/) { $xyz = 1; last SWITCH; }
	       $nothing	= 1;
	   }

       Such constructs are quite frequently used, both because older versions
       of Perl had no official "switch"	statement, and also because the	new
       version described immediately below remains experimental	and can
       sometimes be confusing.

   Switch Statements
       Starting	from Perl 5.10.1 (well,	5.10.0,	but it didn't work right), you
       can say

	   use feature "switch";

       to enable an experimental switch	feature.  This is loosely based	on an
       old version of a	Raku proposal, but it no longer	resembles the Raku
       construct.   You	also get the switch feature whenever you declare that
       your code prefers to run	under a	version	of Perl	that is	5.10 or	later.
       For example:

	   use v5.14;

       Under the "switch" feature, Perl	gains the experimental keywords
       "given",	"when",	"default", "continue", and "break".  Starting from
       Perl 5.16, one can prefix the switch keywords with "CORE::" to access
       the feature without a "use feature" statement.  The keywords "given"
       and "when" are analogous	to "switch" and	"case" in other	languages --
       though "continue" is not	-- so the code in the previous section could
       be rewritten as

	   use v5.10.1;
	   for ($var) {
	       when (/^abc/) { $abc = 1	}
	       when (/^def/) { $def = 1	}
	       when (/^xyz/) { $xyz = 1	}
	       default	     { $nothing	= 1 }
	   }

       The "foreach" is	the non-experimental way to set	a topicalizer.	If you
       wish to use the highly experimental "given", that could be written like
       this:

	   use v5.10.1;
	   given ($var)	{
	       when (/^abc/) { $abc = 1	}
	       when (/^def/) { $def = 1	}
	       when (/^xyz/) { $xyz = 1	}
	       default	     { $nothing	= 1 }
	   }

       As of 5.14, that	can also be written this way:

	   use v5.14;
	   for ($var) {
	       $abc = 1	when /^abc/;
	       $def = 1	when /^def/;
	       $xyz = 1	when /^xyz/;
	       default { $nothing = 1 }
	   }

       Or if you don't care to play it safe, like this:

	   use v5.14;
	   given ($var)	{
	       $abc = 1	when /^abc/;
	       $def = 1	when /^def/;
	       $xyz = 1	when /^xyz/;
	       default { $nothing = 1 }
	   }

       The arguments to	"given"	and "when" are in scalar context, and "given"
       assigns the $_ variable its topic value.

       Exactly what the	EXPR argument to "when"	does is	hard to	describe
       precisely, but in general, it tries to guess what you want done.
       Sometimes it is interpreted as "$_ ~~ EXPR", and	sometimes it is	not.
       It also behaves differently when	lexically enclosed by a	"given"	block
       than it does when dynamically enclosed by a "foreach" loop.  The	rules
       are far too difficult to	understand to be described here.  See
       "Experimental Details on	given and when"	later on.

       Due to an unfortunate bug in how	"given"	was implemented	between	Perl
       5.10 and	5.16, under those implementations the version of $_ governed
       by "given" is merely a lexically	scoped copy of the original, not a
       dynamically scoped alias	to the original, as it would be	if it were a
       "foreach" or under both the original and	the current Raku language
       specification.  This bug	was fixed in Perl 5.18 (and lexicalized	$_
       itself was removed in Perl 5.24).

       If your code still needs	to run on older	versions, stick	to "foreach"
       for your	topicalizer and	you will be less unhappy.

   Goto
       Although	not for	the faint of heart, Perl does support a	"goto"
       statement.  There are three forms: "goto"-LABEL,	"goto"-EXPR, and
       "goto"-&NAME.  A	loop's LABEL is	not actually a valid target for	a
       "goto"; it's just the name of the loop.

       The "goto"-LABEL	form finds the statement labeled with LABEL and
       resumes execution there.	 It may	not be used to go into any construct
       that requires initialization, such as a subroutine or a "foreach" loop.
       It also can't be	used to	go into	a construct that is optimized away.
       It can be used to go almost anywhere else within	the dynamic scope,
       including out of	subroutines, but it's usually better to	use some other
       construct such as "last"	or "die".  The author of Perl has never	felt
       the need	to use this form of "goto" (in Perl, that is--C	is another
       matter).

       The "goto"-EXPR form expects a label name, whose	scope will be resolved
       dynamically.  This allows for computed "goto"s per FORTRAN, but isn't
       necessarily recommended if you're optimizing for	maintainability:

	   goto(("FOO",	"BAR", "GLARCH")[$i]);

       The "goto"-&NAME	form is	highly magical,	and substitutes	a call to the
       named subroutine	for the	currently running subroutine.  This is used by
       "AUTOLOAD()" subroutines	that wish to load another subroutine and then
       pretend that the	other subroutine had been called in the	first place
       (except that any	modifications to @_ in the current subroutine are
       propagated to the other subroutine.)  After the "goto", not even
       "caller()" will be able to tell that this routine was called first.

       In almost all cases like	this, it's usually a far, far better idea to
       use the structured control flow mechanisms of "next", "last", or	"redo"
       instead of resorting to a "goto".  For certain applications, the	catch
       and throw pair of "eval{}" and die() for	exception processing can also
       be a prudent approach.

   The Ellipsis	Statement
       Beginning in Perl 5.12, Perl accepts an ellipsis, ""..."", as a
       placeholder for code that you haven't implemented yet.  When Perl 5.12
       or later	encounters an ellipsis statement, it parses this without
       error, but if and when you should actually try to execute it, Perl
       throws an exception with	the text "Unimplemented":

	   use v5.12;
	   sub unimplemented { ... }
	   eval	{ unimplemented() };
	   if ($@ =~ /^Unimplemented at	/) {
	       say "I found an ellipsis!";
	   }

       You can only use	the elliptical statement to stand in for a complete
       statement.  Syntactically, ""...;"" is a	complete statement, but, as
       with other kinds	of semicolon-terminated	statement, the semicolon may
       be omitted if ""..."" appears immediately before	a closing brace.
       These examples show how the ellipsis works:

	   use v5.12;
	   { ... }
	   sub foo { ... }
	   ...;
	   eval	{ ... };
	   sub somemeth	{
	       my $self	= shift;
	       ...;
	   }
	   $x =	do {
	       my $n;
	       ...;
	       say "Hurrah!";
	       $n;
	   };

       The elliptical statement	cannot stand in	for an expression that is part
       of a larger statement.  These examples of attempts to use an ellipsis
       are syntax errors:

	   use v5.12;

	   print ...;
	   open(my $fh,	">", "/dev/passwd") or ...;
	   if ($condition && ... ) { say "Howdy" };
	   ... if $a > $b;
	   say "Cromulent" if ...;
	   $flub = 5 + ...;

       There are some cases where Perl can't immediately tell the difference
       between an expression and a statement.  For instance, the syntax	for a
       block and an anonymous hash reference constructor look the same unless
       there's something in the	braces to give Perl a hint.  The ellipsis is a
       syntax error if Perl doesn't guess that the "{ ... }" is	a block.
       Inside your block, you can use a	";" before the ellipsis	to denote that
       the "{ ... }" is	a block	and not	a hash reference constructor.

       Note: Some folks	colloquially refer to this bit of punctuation as a
       "yada-yada" or "triple-dot", but	its true name is actually an ellipsis.

   PODs: Embedded Documentation
       Perl has	a mechanism for	intermixing documentation with source code.
       While it's expecting the	beginning of a new statement, if the compiler
       encounters a line that begins with an equal sign	and a word, like this

	   =head1 Here There Be	Pods!

       Then that text and all remaining	text up	through	and including a	line
       beginning with "=cut" will be ignored.  The format of the intervening
       text is described in perlpod.

       This allows you to intermix your	source code and	your documentation
       text freely, as in

	   =item snazzle($)

	   The snazzle() function will behave in the most spectacular
	   form	that you can possibly imagine, not even	excepting
	   cybernetic pyrotechnics.

	   =cut	back to	the compiler, nuff of this pod stuff!

	   sub snazzle($) {
	       my $thingie = shift;
	       .........
	   }

       Note that pod translators should	look at	only paragraphs	beginning with
       a pod directive (it makes parsing easier), whereas the compiler
       actually	knows to look for pod escapes even in the middle of a
       paragraph.  This	means that the following secret	stuff will be ignored
       by both the compiler and	the translators.

	   $a=3;
	   =secret stuff
	    warn "Neither POD nor CODE!?"
	   =cut	back
	   print "got $a\n";

       You probably shouldn't rely upon	the "warn()" being podded out forever.
       Not all pod translators are well-behaved	in this	regard,	and perhaps
       the compiler will become	pickier.

       One may also use	pod directives to quickly comment out a	section	of
       code.

   Plain Old Comments (Not!)
       Perl can	process	line directives, much like the C preprocessor.	Using
       this, one can control Perl's idea of filenames and line numbers in
       error or	warning	messages (especially for strings that are processed
       with "eval()").	The syntax for this mechanism is almost	the same as
       for most	C preprocessors: it matches the	regular	expression

	   # example: '# line 42 "new_filename.plx"'
	   /^\#	  \s*
	     line \s+ (\d+)   \s*
	     (?:\s("?)([^"]+)\g2)? \s*
	    $/x

       with $1 being the line number for the next line,	and $3 being the
       optional	filename (specified with or without quotes).  Note that	no
       whitespace may precede the "#", unlike modern C preprocessors.

       There is	a fairly obvious gotcha	included with the line directive:
       Debuggers and profilers will only show the last source line to appear
       at a particular line number in a	given file.  Care should be taken not
       to cause	line number collisions in code you'd like to debug later.

       Here are	some examples that you should be able to type into your
       command shell:

	   % perl
	   # line 200 "bzzzt"
	   # the '#' on	the previous line must be the first char on line
	   die 'foo';
	   __END__
	   foo at bzzzt	line 201.

	   % perl
	   # line 200 "bzzzt"
	   eval	qq[\n#line 2001	""\ndie	'foo'];	print $@;
	   __END__
	   foo at - line 2001.

	   % perl
	   eval	qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
	   __END__
	   foo at foo bar line 200.

	   % perl
	   # line 345 "goop"
	   eval	"\n#line " . __LINE__ .	' "' . __FILE__	."\"\ndie 'foo'";
	   print $@;
	   __END__
	   foo at goop line 345.

   Experimental	Details	on given and when
       As previously mentioned,	the "switch" feature is	considered highly
       experimental; it	is subject to change with little notice.  In
       particular, "when" has tricky behaviours	that are expected to change to
       become less tricky in the future.  Do not rely upon its current
       (mis)implementation.  Before Perl 5.18, "given" also had	tricky
       behaviours that you should still	beware of if your code must run	on
       older versions of Perl.

       Here is a longer	example	of "given":

	   use feature ":5.10";
	   given ($foo)	{
	       when (undef) {
		   say '$foo is	undefined';
	       }
	       when ("foo") {
		   say '$foo is	the string "foo"';
	       }
	       when ([1,3,5,7,9]) {
		   say '$foo is	an odd digit';
		   continue; # Fall through
	       }
	       when ($_	< 100) {
		   say '$foo is	numerically less than 100';
	       }
	       when (\&complicated_check) {
		   say 'a complicated check for	$foo is	true';
	       }
	       default {
		   die q(I don't know what to do with $foo);
	       }
	   }

       Before Perl 5.18, "given(EXPR)" assigned	the value of EXPR to merely a
       lexically scoped	copy (!) of $_,	not a dynamically scoped alias the way
       "foreach" does.	That made it similar to

	       do { my $_ = EXPR; ... }

       except that the block was automatically broken out of by	a successful
       "when" or an explicit "break".  Because it was only a copy, and because
       it was only lexically scoped, not dynamically scoped, you could not do
       the things with it that you are used to in a "foreach" loop.  In
       particular, it did not work for arbitrary function calls	if those
       functions might try to access $_.  Best stick to	"foreach" for that.

       Most of the power comes from the	implicit smartmatching that can
       sometimes apply.	 Most of the time, "when(EXPR)"	is treated as an
       implicit	smartmatch of $_, that is, "$_ ~~ EXPR".  (See "Smartmatch
       Operator" in perlop for more information	on smartmatching.)  But	when
       EXPR is one of the 10 exceptional cases (or things like them) listed
       below, it is used directly as a boolean.

       1.  A user-defined subroutine call or a method invocation.

       2.  A regular expression	match in the form of "/REGEX/",	"$foo =~
	   /REGEX/", or	"$foo =~ EXPR".	 Also, a negated regular expression
	   match in the	form "!/REGEX/", "$foo !~ /REGEX/", or "$foo !~	EXPR".

       3.  A smart match that uses an explicit "~~" operator, such as "EXPR ~~
	   EXPR".

	   NOTE: You will often	have to	use "$c	~~ $_" because the default
	   case	uses "$_ ~~ $c"	, which	is frequently the opposite of what you
	   want.

       4.  A boolean comparison	operator such as "$_ < 10" or "$x eq "abc"".
	   The relational operators that this applies to are the six numeric
	   comparisons ("<", ">", "<=",	">=", "==", and	"!="), and the six
	   string comparisons ("lt", "gt", "le", "ge", "eq", and "ne").

       5.  At least the	three builtin functions	"defined(...)",	"exists(...)",
	   and "eof(...)".  We might someday add more of these later if	we
	   think of them.

       6.  A negated expression, whether "!(EXPR)" or "not(EXPR)", or a
	   logical exclusive-or, "(EXPR1) xor (EXPR2)".	 The bitwise versions
	   ("~"	and "^") are not included.

       7.  A filetest operator,	with exactly 4 exceptions: "-s", "-M", "-A",
	   and "-C", as	these return numerical values, not boolean ones.  The
	   "-z"	filetest operator is not included in the exception list.

       8.  The ".." and	"..." flip-flop	operators.  Note that the "..."	flip-
	   flop	operator is completely different from the "..."	elliptical
	   statement just described.

       In those	8 cases	above, the value of EXPR is used directly as a
       boolean,	so no smartmatching is done.  You may think of "when" as a
       smartsmartmatch.

       Furthermore, Perl inspects the operands of logical operators to decide
       whether to use smartmatching for	each one by applying the above test to
       the operands:

       9.  If EXPR is "EXPR1 &&	EXPR2" or "EXPR1 and EXPR2", the test is
	   applied recursively to both EXPR1 and EXPR2.	 Only if both operands
	   also	pass the test, recursively, will the expression	be treated as
	   boolean.  Otherwise,	smartmatching is used.

       10. If EXPR is "EXPR1 ||	EXPR2",	"EXPR1 // EXPR2", or "EXPR1 or EXPR2",
	   the test is applied recursively to EXPR1 only (which	might itself
	   be a	higher-precedence AND operator,	for example, and thus subject
	   to the previous rule), not to EXPR2.	 If EXPR1 is to	use
	   smartmatching, then EXPR2 also does so, no matter what EXPR2
	   contains.  But if EXPR2 does	not get	to use smartmatching, then the
	   second argument will	not be either.	This is	quite different	from
	   the "&&" case just described, so be careful.

       These rules are complicated, but	the goal is for	them to	do what	you
       want (even if you don't quite understand	why they are doing it).	 For
       example:

	   when	(/^\d+$/ && $_ < 75) { ... }

       will be treated as a boolean match because the rules say	both a regex
       match and an explicit test on $_	will be	treated	as boolean.

       Also:

	   when	([qw(foo bar)] && /baz/) { ... }

       will use	smartmatching because only one of the operands is a boolean:
       the other uses smartmatching, and that wins.

       Further:

	   when	([qw(foo bar)] || /^baz/) { ...	}

       will use	smart matching (only the first operand is considered), whereas

	   when	(/^baz/	|| [qw(foo bar)]) { ...	}

       will test only the regex, which causes both operands to be treated as
       boolean.	 Watch out for this one, then, because an arrayref is always a
       true value, which makes it effectively redundant.  Not a	good idea.

       Tautologous boolean operators are still going to	be optimized away.
       Don't be	tempted	to write

	   when	("foo" or "bar") { ... }

       This will optimize down to "foo", so "bar" will never be	considered
       (even though the	rules say to use a smartmatch on "foo").  For an
       alternation like	this, an array ref will	work, because this will
       instigate smartmatching:

	   when	([qw(foo bar)] { ... }

       This is somewhat	equivalent to the C-style switch statement's
       fallthrough functionality (not to be confused with Perl's fallthrough
       functionality--see below), wherein the same block is used for several
       "case" statements.

       Another useful shortcut is that,	if you use a literal array or hash as
       the argument to "given",	it is turned into a reference.	So
       "given(@foo)" is	the same as "given(\@foo)", for	example.

       "default" behaves exactly like "when(1 == 1)", which is to say that it
       always matches.

       Breaking	out

       You can use the "break" keyword to break	out of the enclosing "given"
       block.  Every "when" block is implicitly	ended with a "break".

       Fall-through

       You can use the "continue" keyword to fall through from one case	to the
       next immediate "when" or	"default":

	   given($foo) {
	       when (/x/) { say	'$foo contains an x'; continue }
	       when (/y/) { say	'$foo contains a y'	       }
	       default	  { say	'$foo does not contain a y'    }
	   }

       Return value

       When a "given" statement	is also	a valid	expression (for	example, when
       it's the	last statement of a block), it evaluates to:

       o   An empty list as soon as an explicit	"break"	is encountered.

       o   The value of	the last evaluated expression of the successful
	   "when"/"default" clause, if there happens to	be one.

       o   The value of	the last evaluated expression of the "given" block if
	   no condition	is true.

       In both last cases, the last expression is evaluated in the context
       that was	applied	to the "given" block.

       Note that, unlike "if" and "unless", failed "when" statements always
       evaluate	to an empty list.

	   my $price = do {
	       given ($item) {
		   when	(["pear", "apple"]) { 1	}
		   break when "vote";	   # My	vote cannot be bought
		   1e10	 when /Mona Lisa/;
		   "unknown";
	       }
	   };

       Currently, "given" blocks can't always be used as proper	expressions.
       This may	be addressed in	a future version of Perl.

       Switching in a loop

       Instead of using	"given()", you can use a "foreach()" loop.  For
       example,	here's one way to count	how many times a particular string
       occurs in an array:

	   use v5.10.1;
	   my $count = 0;
	   for (@array)	{
	       when ("foo") { ++$count }
	   }
	   print "\@array contains $count copies of 'foo'\n";

       Or in a more recent version:

	   use v5.14;
	   my $count = 0;
	   for (@array)	{
	       ++$count	when "foo";
	   }
	   print "\@array contains $count copies of 'foo'\n";

       At the end of all "when"	blocks,	there is an implicit "next".  You can
       override	that with an explicit "last" if	you're interested in only the
       first match alone.

       This doesn't work if you	explicitly specify a loop variable, as in "for
       $item (@array)".	 You have to use the default variable $_.

       Differences from	Raku

       The Perl	5 smartmatch and "given"/"when"	constructs are not compatible
       with their Raku analogues.  The most visible difference and least
       important difference is that, in	Perl 5,	parentheses are	required
       around the argument to "given()"	and "when()" (except when this last
       one is used as a	statement modifier).  Parentheses in Raku are always
       optional	in a control construct such as "if()", "while()", or "when()";
       they can't be made optional in Perl 5 without a great deal of potential
       confusion, because Perl 5 would parse the expression

	   given $foo {
	       ...
	   }

       as though the argument to "given" were an element of the	hash %foo,
       interpreting the	braces as hash-element syntax.

       However,	their are many,	many other differences.	 For example, this
       works in	Perl 5:

	   use v5.12;
	   my @primary = ("red", "blue", "green");

	   if (@primary	~~ "red") {
	       say "primary smartmatches red";
	   }

	   if ("red" ~~	@primary) {
	       say "red	smartmatches primary";
	   }

	   say "that's all, folks!";

       But it doesn't work at all in Raku.  Instead, you should	use the
       (parallelizable)	"any" operator:

	  if any(@primary) eq "red" {
	      say "primary smartmatches	red";
	  }

	  if "red" eq any(@primary) {
	      say "red smartmatches primary";
	  }

       The table of smartmatches in "Smartmatch	Operator" in perlop is not
       identical to that proposed by the Raku specification, mainly due	to
       differences between Raku's and Perl 5's data models, but	also because
       the Raku	spec has changed since Perl 5 rushed into early	adoption.

       In Raku,	"when()" will always do	an implicit smartmatch with its
       argument, while in Perl 5 it is convenient (albeit potentially
       confusing) to suppress this implicit smartmatch in various rather
       loosely-defined situations, as roughly outlined above.  (The difference
       is largely because Perl 5 does not have,	even internally, a boolean
       type.)

perl v5.34.0			  2021-05-04			    PERLSYN(1)

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