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PERLFAQ7(1)	       Perl Programmers	Reference Guide		   PERLFAQ7(1)

NAME
       perlfaq7	- General Perl Language	Issues

VERSION
       version 5.20200523

DESCRIPTION
       This section deals with general Perl language issues that don't clearly
       fit into	any of the other sections.

   Can I get a BNF/yacc/RE for the Perl	language?
       There is	no BNF,	but you	can paw	your way through the yacc grammar in
       perly.y in the source distribution if you're particularly brave.	The
       grammar relies on very smart tokenizing code, so	be prepared to venture
       into toke.c as well.

       In the words of Chaim Frenkel: "Perl's grammar can not be reduced to
       BNF.  The work of parsing perl is distributed between yacc, the lexer,
       smoke and mirrors."

   What	are all	these $@%&* punctuation	signs, and how do I know when to use
       them?
       They are	type specifiers, as detailed in	perldata:

	   $ for scalar	values (number,	string or reference)
	   @ for arrays
	   % for hashes	(associative arrays)
	   & for subroutines (aka functions, procedures, methods)
	   * for all types of that symbol name.	In version 4 you used them like
	     pointers, but in modern perls you can just	use references.

       There are a couple of other symbols that	you're likely to encounter
       that aren't really type specifiers:

	   <> are used for inputting a record from a filehandle.
	   \  takes a reference	to something.

       Note that <FILE>	is neither the type specifier for files	nor the	name
       of the handle. It is the	"<>" operator applied to the handle FILE. It
       reads one line (well, record--see "$/" in perlvar) from the handle FILE
       in scalar context, or all lines in list context.	When performing	open,
       close, or any other operation besides "<>" on files, or even when
       talking about the handle, do not	use the	brackets. These	are correct:
       "eof(FH)", "seek(FH, 0, 2)" and "copying	from STDIN to FILE".

   Do I	always/never have to quote my strings or use semicolons	and commas?
       Normally, a bareword doesn't need to be quoted, but in most cases
       probably	should be (and must be under "use strict"). But	a hash key
       consisting of a simple word and the left-hand operand to	the "=>"
       operator	both count as though they were quoted:

	   This			   is like this
	   ------------		   ---------------
	   $foo{line}		   $foo{'line'}
	   bar => stuff		   'bar' => stuff

       The final semicolon in a	block is optional, as is the final comma in a
       list. Good style	(see perlstyle)	says to	put them in except for one-
       liners:

	   if ($whoops)	{ exit 1 }
	   my @nums = (1, 2, 3);

	   if ($whoops)	{
	       exit 1;
	   }

	   my @lines = (
	       "There Beren came from mountains	cold",
	       "And lost he wandered under leaves",
	   );

   How do I skip some return values?
       One way is to treat the return values as	a list and index into it:

	   $dir	= (getpwnam($user))[7];

       Another way is to use undef as an element on the	left-hand-side:

	   ($dev, $ino,	undef, undef, $uid, $gid) = stat($file);

       You can also use	a list slice to	select only the	elements that you
       need:

	   ($dev, $ino,	$uid, $gid) = (	stat($file) )[0,1,4,5];

   How do I temporarily	block warnings?
       If you are running Perl 5.6.0 or	better,	the "use warnings" pragma
       allows fine control of what warnings are	produced.  See perllexwarn for
       more details.

	   {
	       no warnings;	     # temporarily turn	off warnings
	       $x = $y + $z;	     # I know these might be undef
	   }

       Additionally, you can enable and	disable	categories of warnings.	 You
       turn off	the categories you want	to ignore and you can still get	other
       categories of warnings. See perllexwarn for the complete	details,
       including the category names and	hierarchy.

	   {
	       no warnings 'uninitialized';
	       $x = $y + $z;
	   }

       If you have an older version of Perl, the $^W variable (documented in
       perlvar)	controls runtime warnings for a	block:

	   {
	       local $^W = 0;	     # temporarily turn	off warnings
	       $x = $y + $z;	     # I know these might be undef
	   }

       Note that like all the punctuation variables, you cannot	currently use
       my() on $^W, only local().

   What's an extension?
       An extension is a way of	calling	compiled C code	from Perl. Reading
       perlxstut is a good place to learn more about extensions.

   Why do Perl operators have different	precedence than	C operators?
       Actually, they don't. All C operators that Perl copies have the same
       precedence in Perl as they do in	C. The problem is with operators that
       C doesn't have, especially functions that give a	list context to
       everything on their right, eg. print, chmod, exec, and so on. Such
       functions are called "list operators" and appear	as such	in the
       precedence table	in perlop.

       A common	mistake	is to write:

	   unlink $file	|| die "snafu";

       This gets interpreted as:

	   unlink ($file || die	"snafu");

       To avoid	this problem, either put in extra parentheses or use the super
       low precedence "or" operator:

	   (unlink $file) || die "snafu";
	   unlink $file	or die "snafu";

       The "English" operators ("and", "or", "xor", and	"not") deliberately
       have precedence lower than that of list operators for just such
       situations as the one above.

       Another operator	with surprising	precedence is exponentiation. It binds
       more tightly even than unary minus, making "-2**2" produce a negative
       four and	not a positive one. It is also right-associating, meaning that
       "2**3**2" is two	raised to the ninth power, not eight squared.

       Although	it has the same	precedence as in C, Perl's "?:"	operator
       produces	an lvalue. This	assigns	$x to either $if_true or $if_false,
       depending on the	trueness of $maybe:

	   ($maybe ? $if_true :	$if_false) = $x;

   How do I declare/create a structure?
       In general, you don't "declare" a structure. Just use a (probably
       anonymous) hash reference. See perlref and perldsc for details.	Here's
       an example:

	   $person = {};		   # new anonymous hash
	   $person->{AGE}  = 24;	   # set field AGE to 24
	   $person->{NAME} = "Nat";	   # set field NAME to "Nat"

       If you're looking for something a bit more rigorous, try	perlootut.

   How do I create a module?
       perlnewmod is a good place to start, ignore the bits about uploading to
       CPAN if you don't want to make your module publicly available.

       ExtUtils::ModuleMaker and Module::Starter are also good places to
       start. Many CPAN	authors	now use	Dist::Zilla to automate	as much	as
       possible.

       Detailed	documentation about modules can	be found at: perlmod,
       perlmodlib, perlmodstyle.

       If you need to include C	code or	C library interfaces use h2xs. h2xs
       will create the module distribution structure and the initial interface
       files.  perlxs and perlxstut explain the	details.

   How do I adopt or take over a module	already	on CPAN?
       Ask the current maintainer to make you a	co-maintainer or transfer the
       module to you.

       If you can not reach the	author for some	reason contact the PAUSE
       admins at modules@perl.org who may be able to help, but each case is
       treated separately.

       o   Get a login for the Perl Authors Upload Server (PAUSE) if you don't
	   already have	one: <http://pause.perl.org>

       o   Write to modules@perl.org explaining	what you did to	contact	the
	   current maintainer. The PAUSE admins	will also try to reach the
	   maintainer.

       o   Post	a public message in a heavily trafficked site announcing your
	   intention to	take over the module.

       o   Wait	a bit. The PAUSE admins	don't want to act too quickly in case
	   the current maintainer is on	holiday. If there's no response	to
	   private communication or the	public post, a PAUSE admin can
	   transfer it to you.

   How do I create a class?
       (contributed by brian d foy)

       In Perl,	a class	is just	a package, and methods are just	subroutines.
       Perl doesn't get	more formal than that and lets you set up the package
       just the	way that you like it (that is, it doesn't set up anything for
       you).

       See also	perlootut, a tutorial that covers class	creation, and perlobj.

   How can I tell if a variable	is tainted?
       You can use the tainted() function of the Scalar::Util module,
       available from CPAN (or included	with Perl since	release	5.8.0).	 See
       also "Laundering	and Detecting Tainted Data" in perlsec.

   What's a closure?
       Closures	are documented in perlref.

       Closure is a computer science term with a precise but hard-to-explain
       meaning.	Usually, closures are implemented in Perl as anonymous
       subroutines with	lasting	references to lexical variables	outside	their
       own scopes. These lexicals magically refer to the variables that	were
       around when the subroutine was defined (deep binding).

       Closures	are most often used in programming languages where you can
       have the	return value of	a function be itself a function, as you	can in
       Perl. Note that some languages provide anonymous	functions but are not
       capable of providing proper closures: the Python	language, for example.
       For more	information on closures, check out any textbook	on functional
       programming. Scheme is a	language that not only supports	but encourages
       closures.

       Here's a	classic	non-closure function-generating	function:

	   sub add_function_generator {
	       return sub { shift() + shift() };
	   }

	   my $add_sub = add_function_generator();
	   my $sum = $add_sub->(4,5);		     # $sum is 9 now.

       The anonymous subroutine	returned by add_function_generator() isn't
       technically a closure because it	refers to no lexicals outside its own
       scope. Using a closure gives you	a function template with some
       customization slots left	out to be filled later.

       Contrast	this with the following	make_adder() function, in which	the
       returned	anonymous function contains a reference	to a lexical variable
       outside the scope of that function itself. Such a reference requires
       that Perl return	a proper closure, thus locking in for all time the
       value that the lexical had when the function was	created.

	   sub make_adder {
	       my $addpiece = shift;
	       return sub { shift() + $addpiece	};
	   }

	   my $f1 = make_adder(20);
	   my $f2 = make_adder(555);

       Now "$f1->($n)" is always 20 plus whatever $n you pass in, whereas
       "$f2->($n)" is always 555 plus whatever $n you pass in. The $addpiece
       in the closure sticks around.

       Closures	are often used for less	esoteric purposes. For example,	when
       you want	to pass	in a bit of code into a	function:

	   my $line;
	   timeout( 30,	sub { $line = <STDIN> }	);

       If the code to execute had been passed in as a string, '$line =
       <STDIN>', there would have been no way for the hypothetical timeout()
       function	to access the lexical variable $line back in its caller's
       scope.

       Another use for a closure is to make a variable private to a named
       subroutine, e.g.	a counter that gets initialized	at creation time of
       the sub and can only be modified	from within the	sub.  This is
       sometimes used with a BEGIN block in package files to make sure a
       variable	doesn't	get meddled with during	the lifetime of	the package:

	   BEGIN {
	       my $id =	0;
	       sub next_id { ++$id }
	   }

       This is discussed in more detail	in perlsub; see	the entry on
       Persistent Private Variables.

   What	is variable suicide and	how can	I prevent it?
       This problem was	fixed in perl 5.004_05,	so preventing it means
       upgrading your version of perl. ;)

       Variable	suicide	is when	you (temporarily or permanently) lose the
       value of	a variable. It is caused by scoping through my() and local()
       interacting with	either closures	or aliased foreach() iterator
       variables and subroutine	arguments. It used to be easy to inadvertently
       lose a variable's value this way, but now it's much harder. Take	this
       code:

	   my $f = 'foo';
	   sub T {
	       while ($i++ < 3)	{ my $f	= $f; $f .= "bar"; print $f, "\n" }
	   }

	   T;
	   print "Finally $f\n";

       If you are experiencing variable	suicide, that "my $f" in the
       subroutine doesn't pick up a fresh copy of the $f whose value is	'foo'.
       The output shows	that inside the	subroutine the value of	$f leaks
       through when it shouldn't, as in	this output:

	   foobar
	   foobarbar
	   foobarbarbar
	   Finally foo

       The $f that has "bar" added to it three times should be a new $f	"my
       $f" should create a new lexical variable	each time through the loop.
       The expected output is:

	   foobar
	   foobar
	   foobar
	   Finally foo

   How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
       You need	to pass	references to these objects. See "Pass by Reference"
       in perlsub for this particular question,	and perlref for	information on
       references.

       Passing Variables and Functions
	   Regular variables and functions are quite easy to pass: just	pass
	   in a	reference to an	existing or anonymous variable or function:

	       func( \$some_scalar );

	       func( \@some_array  );
	       func( [ 1 .. 10 ]   );

	       func( \%some_hash   );
	       func( { this => 10, that	=> 20 }	  );

	       func( \&some_func   );
	       func( sub { $_[0] ** $_[1] }   );

       Passing Filehandles
	   As of Perl 5.6, you can represent filehandles with scalar variables
	   which you treat as any other	scalar.

	       open my $fh, $filename or die "Cannot open $filename! $!";
	       func( $fh );

	       sub func	{
		   my $passed_fh = shift;

		   my $line = <$passed_fh>;
	       }

	   Before Perl 5.6, you	had to use the *FH or "\*FH" notations.	 These
	   are "typeglobs"--see	"Typeglobs and Filehandles" in perldata	and
	   especially "Pass by Reference" in perlsub for more information.

       Passing Regexes
	   Here's an example of	how to pass in a string	and a regular
	   expression for it to	match against. You construct the pattern with
	   the "qr//" operator:

	       sub compare {
		   my ($val1, $regex) =	@_;
		   my $retval =	$val1 =~ /$regex/;
		   return $retval;
	       }
	       $match =	compare("old McDonald",	qr/d.*D/i);

       Passing Methods
	   To pass an object method into a subroutine, you can do this:

	       call_a_lot(10, $some_obj, "methname")
	       sub call_a_lot {
		   my ($count, $widget,	$trick)	= @_;
		   for (my $i =	0; $i <	$count;	$i++) {
		       $widget->$trick();
		   }
	       }

	   Or, you can use a closure to	bundle up the object, its method call,
	   and arguments:

	       my $whatnot = sub { $some_obj->obfuscate(@args) };
	       func($whatnot);
	       sub func	{
		   my $code = shift;
		   &$code();
	       }

	   You could also investigate the can()	method in the UNIVERSAL	class
	   (part of the	standard perl distribution).

   How do I create a static variable?
       (contributed by brian d foy)

       In Perl 5.10, declare the variable with "state".	The "state"
       declaration creates the lexical variable	that persists between calls to
       the subroutine:

	   sub counter { state $count =	1; $count++ }

       You can fake a static variable by using a lexical variable which	goes
       out of scope. In	this example, you define the subroutine	"counter", and
       it uses the lexical variable $count. Since you wrap this	in a BEGIN
       block, $count is	defined	at compile-time, but also goes out of scope at
       the end of the BEGIN block. The BEGIN block also	ensures	that the
       subroutine and the value	it uses	is defined at compile-time so the
       subroutine is ready to use just like any	other subroutine, and you can
       put this	code in	the same place as other	subroutines in the program
       text (i.e. at the end of	the code, typically). The subroutine "counter"
       still has a reference to	the data, and is the only way you can access
       the value (and each time	you do,	you increment the value).  The data in
       chunk of	memory defined by $count is private to "counter".

	   BEGIN {
	       my $count = 1;
	       sub counter { $count++ }
	   }

	   my $start = counter();

	   ....	# code that calls counter();

	   my $end = counter();

       In the previous example,	you created a function-private variable
       because only one	function remembered its	reference. You could define
       multiple	functions while	the variable is	in scope, and each function
       can share the "private" variable. It's not really "static" because you
       can access it outside the function while	the lexical variable is	in
       scope, and even create references to it.	In this	example,
       "increment_count" and "return_count" share the variable.	One function
       adds to the value and the other simply returns the value.  They can
       both access $count, and since it	has gone out of	scope, there is	no
       other way to access it.

	   BEGIN {
	       my $count = 1;
	       sub increment_count { $count++ }
	       sub return_count	   { $count }
	   }

       To declare a file-private variable, you still use a lexical variable.
       A file is also a	scope, so a lexical variable defined in	the file
       cannot be seen from any other file.

       See "Persistent Private Variables" in perlsub for more information.
       The discussion of closures in perlref may help you even though we did
       not use anonymous subroutines in	this answer. See "Persistent Private
       Variables" in perlsub for details.

   What's the difference between dynamic and lexical (static) scoping? Between
       local() and my()?
       "local($x)" saves away the old value of the global variable $x and
       assigns a new value for the duration of the subroutine which is visible
       in other	functions called from that subroutine. This is done at run-
       time, so	is called dynamic scoping. local() always affects global
       variables, also called package variables	or dynamic variables.

       "my($x)"	creates	a new variable that is only visible in the current
       subroutine. This	is done	at compile-time, so it is called lexical or
       static scoping. my() always affects private variables, also called
       lexical variables or (improperly) static(ly scoped) variables.

       For instance:

	   sub visible {
	       print "var has value $var\n";
	   }

	   sub dynamic {
	       local $var = 'local';	# new temporary	value for the still-global
	       visible();	       #   variable called $var
	   }

	   sub lexical {
	       my $var = 'private';    # new private variable, $var
	       visible();	       # (invisible outside of sub scope)
	   }

	   $var	= 'global';

	   visible();		   # prints global
	   dynamic();		   # prints local
	   lexical();		   # prints global

       Notice how at no	point does the value "private" get printed. That's
       because $var only has that value	within the block of the	lexical()
       function, and it	is hidden from the called subroutine.

       In summary, local() doesn't make	what you think of as private, local
       variables. It gives a global variable a temporary value.	my() is	what
       you're looking for if you want private variables.

       See "Private Variables via my()"	in perlsub and "Temporary Values via
       local()"	in perlsub for excruciating details.

   How can I access a dynamic variable while a similarly named lexical is in
       scope?
       If you know your	package, you can just mention it explicitly, as	in
       $Some_Pack::var.	Note that the notation $::var is not the dynamic $var
       in the current package, but rather the one in the "main"	package, as
       though you had written $main::var.

	   use vars '$var';
	   local $var =	"global";
	   my	 $var =	"lexical";

	   print "lexical is $var\n";
	   print "global  is $main::var\n";

       Alternatively you can use the compiler directive	our() to bring a
       dynamic variable	into the current lexical scope.

	   require 5.006; # our() did not exist	before 5.6
	   use vars '$var';

	   local $var =	"global";
	   my $var    =	"lexical";

	   print "lexical is $var\n";

	   {
	       our $var;
	       print "global  is $var\n";
	   }

   What's the difference between deep and shallow binding?
       In deep binding,	lexical	variables mentioned in anonymous subroutines
       are the same ones that were in scope when the subroutine	was created.
       In shallow binding, they	are whichever variables	with the same names
       happen to be in scope when the subroutine is called. Perl always	uses
       deep binding of lexical variables (i.e.,	those created with my()).
       However,	dynamic	variables (aka global, local, or package variables)
       are effectively shallowly bound.	Consider this just one more reason not
       to use them. See	the answer to "What's a	closure?".

   Why doesn't "my($foo) = <$fh>;" work	right?
       "my()" and "local()" give list context to the right hand	side of	"=".
       The <$fh> read operation, like so many of Perl's	functions and
       operators, can tell which context it was	called in and behaves
       appropriately. In general, the scalar() function	can help.  This
       function	does nothing to	the data itself	(contrary to popular myth) but
       rather tells its	argument to behave in whatever its scalar fashion is.
       If that function	doesn't	have a defined scalar behavior,	this of	course
       doesn't help you	(such as with sort()).

       To enforce scalar context in this particular case, however, you need
       merely omit the parentheses:

	   local($foo) = <$fh>;	       # WRONG
	   local($foo) = scalar(<$fh>);	  # ok
	   local $foo  = <$fh>;	       # right

       You should probably be using lexical variables anyway, although the
       issue is	the same here:

	   my($foo) = <$fh>;	# WRONG
	   my $foo  = <$fh>;	# right

   How do I redefine a builtin function, operator, or method?
       Why do you want to do that? :-)

       If you want to override a predefined function, such as open(), then
       you'll have to import the new definition	from a different module. See
       "Overriding Built-in Functions" in perlsub.

       If you want to overload a Perl operator,	such as	"+" or "**", then
       you'll want to use the "use overload" pragma, documented	in overload.

       If you're talking about obscuring method	calls in parent	classes, see
       "Overriding methods and method resolution" in perlootut.

   What's the difference between calling a function as &foo and	foo()?
       (contributed by brian d foy)

       Calling a subroutine as &foo with no trailing parentheses ignores the
       prototype of "foo" and passes it	the current value of the argument
       list, @_. Here's	an example; the	"bar" subroutine calls &foo, which
       prints its arguments list:

	   sub foo { print "Args in foo	are: @_\n"; }

	   sub bar { &foo; }

	   bar(	"a", "b", "c" );

       When you	call "bar" with	arguments, you see that	"foo" got the same @_:

	   Args	in foo are: a b	c

       Calling the subroutine with trailing parentheses, with or without
       arguments, does not use the current @_. Changing	the example to put
       parentheses after the call to "foo" changes the program:

	   sub foo { print "Args in foo	are: @_\n"; }

	   sub bar { &foo(); }

	   bar(	"a", "b", "c" );

       Now the output shows that "foo" doesn't get the @_ from its caller.

	   Args	in foo are:

       However,	using "&" in the call still overrides the prototype of "foo"
       if present:

	   sub foo ($$$) { print "Args infoo are: @_\n"; }

	   sub bar_1 { &foo; }
	   sub bar_2 { &foo(); }
	   sub bar_3 { foo( $_[0], $_[1], $_[2]	); }
	   # sub bar_4 { foo();	}
	   # bar_4 doesn't compile: "Not enough	arguments for main::foo	at ..."

	   bar_1( "a", "b", "c"	);
	   # Args in foo are: a	b c

	   bar_2( "a", "b", "c"	);
	   # Args in foo are:

	   bar_3( "a", "b", "c"	);
	   # Args in foo are: a	b c

       The main	use of the @_ pass-through feature is to write subroutines
       whose main job it is to call other subroutines for you. For further
       details,	see perlsub.

   How do I create a switch or case statement?
       There is	a given/when statement in Perl,	but it is experimental and
       likely to change	in future. See perlsyn for more	details.

       The general answer is to	use a CPAN module such as Switch::Plain:

	   use Switch::Plain;
	   sswitch($variable_holding_a_string) {
	       case 'first': { }
	       case 'second': {	}
	       default:	{ }
	   }

       or for more complicated comparisons, "if-elsif-else":

	   for ($variable_to_test) {
	       if    (/pat1/)  { }     # do something
	       elsif (/pat2/)  { }     # do something else
	       elsif (/pat3/)  { }     # do something else
	       else	       { }     # default
	   }

       Here's a	simple example of a switch based on pattern matching, lined up
       in a way	to make	it look	more like a switch statement.  We'll do	a
       multiway	conditional based on the type of reference stored in
       $whatchamacallit:

	   SWITCH: for (ref $whatchamacallit) {

	       /^$/	      && die "not a reference";

	       /SCALAR/	      && do {
			       print_scalar($$ref);
			       last SWITCH;
			     };

	       /ARRAY/	      && do {
			       print_array(@$ref);
			       last SWITCH;
			     };

	       /HASH/	     &&	do {
			       print_hash(%$ref);
			       last SWITCH;
			     };

	       /CODE/	     &&	do {
			       warn "can't print function ref";
			       last SWITCH;
			     };

	       # DEFAULT

	       warn "User defined type skipped";

	   }

       See perlsyn for other examples in this style.

       Sometimes you should change the positions of the	constant and the
       variable.  For example, let's say you wanted to test which of many
       answers you were	given, but in a	case-insensitive way that also allows
       abbreviations.  You can use the following technique if the strings all
       start with different characters or if you want to arrange the matches
       so that one takes precedence over another, as "SEND" has	precedence
       over "STOP" here:

	   chomp($answer = <>);
	   if	 ("SEND"  =~ /^\Q$answer/i) { print "Action is send\n"	}
	   elsif ("STOP"  =~ /^\Q$answer/i) { print "Action is stop\n"	}
	   elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n"	}
	   elsif ("LIST"  =~ /^\Q$answer/i) { print "Action is list\n"	}
	   elsif ("EDIT"  =~ /^\Q$answer/i) { print "Action is edit\n"	}

       A totally different approach is to create a hash	of function
       references.

	   my %commands	= (
	       "happy" => \&joy,
	       "sad",  => \&sullen,
	       "done"  => sub {	die "See ya!" },
	       "mad"   => \&angry,
	   );

	   print "How are you? ";
	   chomp($string = <STDIN>);
	   if ($commands{$string}) {
	       $commands{$string}->();
	   } else {
	       print "No such command: $string\n";
	   }

       Starting	from Perl 5.8, a source	filter module, "Switch", can also be
       used to get switch and case. Its	use is now discouraged,	because	it's
       not fully compatible with the native switch of Perl 5.10, and because,
       as it's implemented as a	source filter, it doesn't always work as
       intended	when complex syntax is involved.

   How can I catch accesses to undefined variables, functions, or methods?
       The AUTOLOAD method, discussed in "Autoloading" in perlsub lets you
       capture calls to	undefined functions and	methods.

       When it comes to	undefined variables that would trigger a warning under
       "use warnings", you can promote the warning to an error.

	   use warnings	FATAL => qw(uninitialized);

   Why can't a method included in this same file be found?
       Some possible reasons: your inheritance is getting confused, you've
       misspelled the method name, or the object is of the wrong type. Check
       out perlootut for details about any of the above	cases. You may also
       use "print ref($object)"	to find	out the	class $object was blessed
       into.

       Another possible	reason for problems is that you've used	the indirect
       object syntax (eg, "find	Guru "Samy"") on a class name before Perl has
       seen that such a	package	exists.	It's wisest to make sure your packages
       are all defined before you start	using them, which will be taken	care
       of if you use the "use" statement instead of "require". If not, make
       sure to use arrow notation (eg.,	"Guru->find("Samy")") instead. Object
       notation	is explained in	perlobj.

       Make sure to read about creating	modules	in perlmod and the perils of
       indirect	objects	in "Method Invocation" in perlobj.

   How can I find out my current or calling package?
       (contributed by brian d foy)

       To find the package you are currently in, use the special literal
       "__PACKAGE__", as documented in perldata. You can only use the special
       literals	as separate tokens, so you can't interpolate them into strings
       like you	can with variables:

	   my $current_package = __PACKAGE__;
	   print "I am in package $current_package\n";

       If you want to find the package calling your code, perhaps to give
       better diagnostics as Carp does,	use the	"caller" built-in:

	   sub foo {
	       my @args	= ...;
	       my( $package, $filename,	$line )	= caller;

	       print "I	was called from	package	$package\n";
	       );

       By default, your	program	starts in package "main", so you will always
       be in some package.

       This is different from finding out the package an object	is blessed
       into, which might not be	the current package. For that, use "blessed"
       from Scalar::Util, part of the Standard Library since Perl 5.8:

	   use Scalar::Util qw(blessed);
	   my $object_package =	blessed( $object );

       Most of the time, you shouldn't care what package an object is blessed
       into, however, as long as it claims to inherit from that	class:

	   my $is_right_class =	eval { $object->isa( $package )	}; # true or false

       And, with Perl 5.10 and later, you don't	have to	check for an
       inheritance to see if the object	can handle a role. For that, you can
       use "DOES", which comes from "UNIVERSAL":

	   my $class_does_it = eval { $object->DOES( $role ) };	# true or false

       You can safely replace "isa" with "DOES"	(although the converse is not
       true).

   How can I comment out a large block of Perl code?
       (contributed by brian d foy)

       The quick-and-dirty way to comment out more than	one line of Perl is to
       surround	those lines with Pod directives. You have to put these
       directives at the beginning of the line and somewhere where Perl
       expects a new statement (so not in the middle of	statements like	the
       "#" comments). You end the comment with "=cut", ending the Pod section:

	   =pod

	   my $object =	NotGonnaHappen->new();

	   ignored_sub();

	   $wont_be_assigned = 37;

	   =cut

       The quick-and-dirty method only works well when you don't plan to leave
       the commented code in the source. If a Pod parser comes along, your
       multiline comment is going to show up in	the Pod	translation.  A	better
       way hides it from Pod parsers as	well.

       The "=begin" directive can mark a section for a particular purpose.  If
       the Pod parser doesn't want to handle it, it just ignores it. Label the
       comments	with "comment".	End the	comment	using "=end" with the same
       label. You still	need the "=cut"	to go back to Perl code	from the Pod
       comment:

	   =begin comment

	   my $object =	NotGonnaHappen->new();

	   ignored_sub();

	   $wont_be_assigned = 37;

	   =end	comment

	   =cut

       For more	information on Pod, check out perlpod and perlpodspec.

   How do I clear a package?
       Use this	code, provided by Mark-Jason Dominus:

	   sub scrub_package {
	       no strict 'refs';
	       my $pack	= shift;
	       die "Shouldn't delete main package"
		   if $pack eq "" || $pack eq "main";
	       my $stash = *{$pack . '::'}{HASH};
	       my $name;
	       foreach $name (keys %$stash) {
		   my $fullname	= $pack	. '::' . $name;
		   # Get rid of	everything with	that name.
		   undef $$fullname;
		   undef @$fullname;
		   undef %$fullname;
		   undef &$fullname;
		   undef *$fullname;
	       }
	   }

       Or, if you're using a recent release of Perl, you can just use the
       Symbol::delete_package()	function instead.

   How can I use a variable as a variable name?
       Beginners often think they want to have a variable contain the name of
       a variable.

	   $fred    = 23;
	   $varname = "fred";
	   ++$$varname;		# $fred	now 24

       This works sometimes, but it is a very bad idea for two reasons.

       The first reason	is that	this technique only works on global variables.
       That means that if $fred	is a lexical variable created with my()	in the
       above example, the code wouldn't	work at	all: you'd accidentally	access
       the global and skip right over the private lexical altogether. Global
       variables are bad because they can easily collide accidentally and in
       general make for	non-scalable and confusing code.

       Symbolic	references are forbidden under the "use	strict"	pragma.	 They
       are not true references and consequently	are not	reference-counted or
       garbage-collected.

       The other reason	why using a variable to	hold the name of another
       variable	is a bad idea is that the question often stems from a lack of
       understanding of	Perl data structures, particularly hashes. By using
       symbolic	references, you	are just using the package's symbol-table hash
       (like %main::) instead of a user-defined	hash. The solution is to use
       your own	hash or	a real reference instead.

	   $USER_VARS{"fred"} =	23;
	   my $varname = "fred";
	   $USER_VARS{$varname}++;  # not $$varname++

       There we're using the %USER_VARS	hash instead of	symbolic references.
       Sometimes this comes up in reading strings from the user	with variable
       references and wanting to expand	them to	the values of your perl
       program's variables. This is also a bad idea because it conflates the
       program-addressable namespace and the user-addressable one. Instead of
       reading a string	and expanding it to the	actual contents	of your
       program's own variables:

	   $str	= 'this	has a $fred and	$barney	in it';
	   $str	=~ s/(\$\w+)/$1/eeg;	      #	need double eval

       it would	be better to keep a hash around	like %USER_VARS	and have
       variable	references actually refer to entries in	that hash:

	   $str	=~ s/\$(\w+)/$USER_VARS{$1}/g;	 # no /e here at all

       That's faster, cleaner, and safer than the previous approach. Of
       course, you don't need to use a dollar sign. You	could use your own
       scheme to make it less confusing, like bracketed	percent	symbols, etc.

	   $str	= 'this	has a %fred% and %barney% in it';
	   $str	=~ s/%(\w+)%/$USER_VARS{$1}/g;	 # no /e here at all

       Another reason that folks sometimes think they want a variable to
       contain the name	of a variable is that they don't know how to build
       proper data structures using hashes. For	example, let's say they	wanted
       two hashes in their program: %fred and %barney, and that	they wanted to
       use another scalar variable to refer to those by	name.

	   $name = "fred";
	   $$name{WIFE}	= "wilma";     # set %fred

	   $name = "barney";
	   $$name{WIFE}	= "betty";    #	set %barney

       This is still a symbolic	reference, and is still	saddled	with the
       problems	enumerated above. It would be far better to write:

	   $folks{"fred"}{WIFE}	  = "wilma";
	   $folks{"barney"}{WIFE} = "betty";

       And just	use a multilevel hash to start with.

       The only	times that you absolutely must use symbolic references are
       when you	really must refer to the symbol	table. This may	be because
       it's something that one can't take a real reference to, such as a
       format name.  Doing so may also be important for	method calls, since
       these always go through the symbol table	for resolution.

       In those	cases, you would turn off "strict 'refs'" temporarily so you
       can play	around with the	symbol table. For example:

	   @colors = qw(red blue green yellow orange purple violet);
	   for my $name	(@colors) {
	       no strict 'refs';  # renege for the block
	       *$name =	sub { "<FONT COLOR='$name'>@_</FONT>" };
	   }

       All those functions (red(), blue(), green(), etc.) appear to be
       separate, but the real code in the closure actually was compiled	only
       once.

       So, sometimes you might want to use symbolic references to manipulate
       the symbol table	directly. This doesn't matter for formats, handles,
       and subroutines,	because	they are always	global--you can't use my() on
       them.  For scalars, arrays, and hashes, though--and usually for
       subroutines-- you probably only want to use hard	references.

   What	does "bad interpreter" mean?
       (contributed by brian d foy)

       The "bad	interpreter" message comes from	the shell, not perl. The
       actual message may vary depending on your platform, shell, and locale
       settings.

       If you see "bad interpreter - no	such file or directory", the first
       line in your perl script	(the "shebang" line) does not contain the
       right path to perl (or any other	program	capable	of running scripts).
       Sometimes this happens when you move the	script from one	machine	to
       another and each	machine	has a different	path to	perl--/usr/bin/perl
       versus /usr/local/bin/perl for instance.	It may also indicate that the
       source machine has CRLF line terminators	and the	destination machine
       has LF only: the	shell tries to find /usr/bin/perl<CR>, but can't.

       If you see "bad interpreter: Permission denied",	you need to make your
       script executable.

       In either case, you should still	be able	to run the scripts with	perl
       explicitly:

	   % perl script.pl

       If you get a message like "perl:	command	not found", perl is not	in
       your PATH, which	might also mean	that the location of perl is not where
       you expect it so	you need to adjust your	shebang	line.

   Do I	need to	recompile XS modules when there	is a change in the C library?
       (contributed by Alex Beamish)

       If the new version of the C library is ABI-compatible (that's
       Application Binary Interface compatible)	with the version you're
       upgrading from, and if the shared library version didn't	change,	no re-
       compilation should be necessary.

AUTHOR AND COPYRIGHT
       Copyright (c) 1997-2013 Tom Christiansen, Nathan	Torkington, and	other
       authors as noted. All rights reserved.

       This documentation is free; you can redistribute	it and/or modify it
       under the same terms as Perl itself.

       Irrespective of its distribution, all code examples in this file	are
       hereby placed into the public domain. You are permitted and encouraged
       to use this code	in your	own programs for fun or	for profit as you see
       fit. A simple comment in	the code giving	credit would be	courteous but
       is not required.

perl v5.32.0			  2020-06-14			   PERLFAQ7(1)

NAME | VERSION | DESCRIPTION | AUTHOR AND COPYRIGHT

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