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PERLREF(1)	       Perl Programmers	Reference Guide		    PERLREF(1)

NAME
       perlref - Perl references and nested data structures

NOTE
       This is complete	documentation about all	aspects	of references.	For a
       shorter,	tutorial introduction to just the essential features, see
       perlreftut.

DESCRIPTION
       Before release 5	of Perl	it was difficult to represent complex data
       structures, because all references had to be symbolic--and even then it
       was difficult to	refer to a variable instead of a symbol	table entry.
       Perl now	not only makes it easier to use	symbolic references to
       variables, but also lets	you have "hard"	references to any piece	of
       data or code.  Any scalar may hold a hard reference.  Because arrays
       and hashes contain scalars, you can now easily build arrays of arrays,
       arrays of hashes, hashes	of arrays, arrays of hashes of functions, and
       so on.

       Hard references are smart--they keep track of reference counts for you,
       automatically freeing the thing referred	to when	its reference count
       goes to zero.  (Reference counts	for values in self-referential or
       cyclic data structures may not go to zero without a little help;	see
       "Circular References" for a detailed explanation.)  If that thing
       happens to be an	object,	the object is destructed.  See perlobj for
       more about objects.  (In	a sense, everything in Perl is an object, but
       we usually reserve the word for references to objects that have been
       officially "blessed" into a class package.)

       Symbolic	references are names of	variables or other objects, just as a
       symbolic	link in	a Unix filesystem contains merely the name of a	file.
       The *glob notation is something of a symbolic reference.	 (Symbolic
       references are sometimes	called "soft references", but please don't
       call them that; references are confusing	enough without useless
       synonyms.)

       In contrast, hard references are	more like hard links in	a Unix file
       system: They are	used to	access an underlying object without concern
       for what	its (other) name is.  When the word "reference"	is used
       without an adjective, as	in the following paragraph, it is usually
       talking about a hard reference.

       References are easy to use in Perl.  There is just one overriding
       principle: in general, Perl does	no implicit referencing	or
       dereferencing.  When a scalar is	holding	a reference, it	always behaves
       as a simple scalar.  It doesn't magically start being an	array or hash
       or subroutine; you have to tell it explicitly to	do so, by
       dereferencing it.

   Making References
       References can be created in several ways.

       Backslash Operator

       By using	the backslash operator on a variable, subroutine, or value.
       (This works much	like the & (address-of)	operator in C.)	 This
       typically creates another reference to a	variable, because there's
       already a reference to the variable in the symbol table.	 But the
       symbol table reference might go away, and you'll	still have the
       reference that the backslash returned.  Here are	some examples:

	   $scalarref =	\$foo;
	   $arrayref  =	\@ARGV;
	   $hashref   =	\%ENV;
	   $coderef   =	\&handler;
	   $globref   =	\*foo;

       It isn't	possible to create a true reference to an IO handle
       (filehandle or dirhandle) using the backslash operator.	The most you
       can get is a reference to a typeglob, which is actually a complete
       symbol table entry.  But	see the	explanation of the *foo{THING} syntax
       below.  However,	you can	still use type globs and globrefs as though
       they were IO handles.

       Square Brackets

       A reference to an anonymous array can be	created	using square brackets:

	   $arrayref = [1, 2, ['a', 'b', 'c']];

       Here we've created a reference to an anonymous array of three elements
       whose final element is itself a reference to another anonymous array of
       three elements.	(The multidimensional syntax described later can be
       used to access this.  For example, after	the above, "$arrayref->[2][1]"
       would have the value "b".)

       Taking a	reference to an	enumerated list	is not the same	as using
       square brackets--instead	it's the same as creating a list of
       references!

	   @list = (\$a, \@b, \%c);
	   @list = \($a, @b, %c);      # same thing!

       As a special case, "\(@foo)" returns a list of references to the
       contents	of @foo, not a reference to @foo itself.  Likewise for %foo,
       except that the key references are to copies (since the keys are	just
       strings rather than full-fledged	scalars).

       Curly Brackets

       A reference to an anonymous hash	can be created using curly brackets:

	   $hashref = {
	       'Adam'  => 'Eve',
	       'Clyde' => 'Bonnie',
	   };

       Anonymous hash and array	composers like these can be intermixed freely
       to produce as complicated a structure as	you want.  The
       multidimensional	syntax described below works for these too.  The
       values above are	literals, but variables	and expressions	would work
       just as well, because assignment	operators in Perl (even	within local()
       or my())	are executable statements, not compile-time declarations.

       Because curly brackets (braces) are used	for several other things
       including BLOCKs, you may occasionally have to disambiguate braces at
       the beginning of	a statement by putting a "+" or	a "return" in front so
       that Perl realizes the opening brace isn't starting a BLOCK.  The
       economy and mnemonic value of using curlies is deemed worth this
       occasional extra	hassle.

       For example, if you wanted a function to	make a new hash	and return a
       reference to it,	you have these options:

	   sub hashem {	       { @_ } }	  # silently wrong
	   sub hashem {	      +{ @_ } }	  # ok
	   sub hashem {	return { @_ } }	  # ok

       On the other hand, if you want the other	meaning, you can do this:

	   sub showem {	       { @_ } }	  # ambiguous (currently ok,
					  # but	may change)
	   sub showem {	      {; @_ } }	  # ok
	   sub showem {	{ return @_ } }	  # ok

       The leading "+{"	and "{;" always	serve to disambiguate the expression
       to mean either the HASH reference, or the BLOCK.

       Anonymous Subroutines

       A reference to an anonymous subroutine can be created by	using "sub"
       without a subname:

	   $coderef = sub { print "Boink!\n" };

       Note the	semicolon.  Except for the code	inside not being immediately
       executed, a "sub	{}" is not so much a declaration as it is an operator,
       like "do{}" or "eval{}".	 (However, no matter how many times you
       execute that particular line (unless you're in an "eval("...")"),
       $coderef	will still have	a reference to the same	anonymous subroutine.)

       Anonymous subroutines act as closures with respect to my() variables,
       that is,	variables lexically visible within the current scope.  Closure
       is a notion out of the Lisp world that says if you define an anonymous
       function	in a particular	lexical	context, it pretends to	run in that
       context even when it's called outside the context.

       In human	terms, it's a funny way	of passing arguments to	a subroutine
       when you	define it as well as when you call it.	It's useful for
       setting up little bits of code to run later, such as callbacks.	You
       can even	do object-oriented stuff with it, though Perl already provides
       a different mechanism to	do that--see perlobj.

       You might also think of closure as a way	to write a subroutine template
       without using eval().  Here's a small example of	how closures work:

	   sub newprint	{
	       my $x = shift;
	       return sub { my $y = shift; print "$x, $y!\n"; };
	   }
	   $h =	newprint("Howdy");
	   $g =	newprint("Greetings");

	   # Time passes...

	   &$h("world");
	   &$g("earthlings");

       This prints

	   Howdy, world!
	   Greetings, earthlings!

       Note particularly that $x continues to refer to the value passed	into
       newprint() despite "my $x" having gone out of scope by the time the
       anonymous subroutine runs.  That's what a closure is all	about.

       This applies only to lexical variables, by the way.  Dynamic variables
       continue	to work	as they	have always worked.  Closure is	not something
       that most Perl programmers need trouble themselves about	to begin with.

       Constructors

       References are often returned by	special	subroutines called
       constructors.  Perl objects are just references to a special type of
       object that happens to know which package it's associated with.
       Constructors are	just special subroutines that know how to create that
       association.  They do so	by starting with an ordinary reference,	and it
       remains an ordinary reference even while	it's also being	an object.
       Constructors are	often named "new()".  You can call them	indirectly:

	   $objref = new Doggie( Tail => 'short', Ears => 'long' );

       But that	can produce ambiguous syntax in	certain	cases, so it's often
       better to use the direct	method invocation approach:

	   $objref   = Doggie->new(Tail	=> 'short', Ears => 'long');

	   use Term::Cap;
	   $terminal = Term::Cap->Tgetent( { OSPEED => 9600 });

	   use Tk;
	   $main    = MainWindow->new();
	   $menubar = $main->Frame(-relief		=> "raised",
				   -borderwidth		=> 2)

       Autovivification

       References of the appropriate type can spring into existence if you
       dereference them	in a context that assumes they exist.  Because we
       haven't talked about dereferencing yet, we can't	show you any examples
       yet.

       Typeglob	Slots

       A reference can be created by using a special syntax, lovingly known as
       the *foo{THING} syntax.	*foo{THING} returns a reference	to the THING
       slot in *foo (which is the symbol table entry which holds everything
       known as	foo).

	   $scalarref =	*foo{SCALAR};
	   $arrayref  =	*ARGV{ARRAY};
	   $hashref   =	*ENV{HASH};
	   $coderef   =	*handler{CODE};
	   $ioref     =	*STDIN{IO};
	   $globref   =	*foo{GLOB};
	   $formatref =	*foo{FORMAT};
	   $globname  =	*foo{NAME};    # "foo"
	   $pkgname   =	*foo{PACKAGE}; # "main"

       Most of these are self-explanatory, but *foo{IO}	deserves special
       attention.  It returns the IO handle, used for file handles ("open" in
       perlfunc), sockets ("socket" in perlfunc	and "socketpair" in perlfunc),
       and directory handles ("opendir"	in perlfunc).  For compatibility with
       previous	versions of Perl, *foo{FILEHANDLE} is a	synonym	for *foo{IO},
       though it is discouraged, to encourage a	consistent use of one name:
       IO.  On perls between v5.8 and v5.22, it	will issue a deprecation
       warning,	but this deprecation has since been rescinded.

       *foo{THING} returns undef if that particular THING hasn't been used
       yet, except in the case of scalars.  *foo{SCALAR} returns a reference
       to an anonymous scalar if $foo hasn't been used yet.  This might	change
       in a future release.

       *foo{NAME} and *foo{PACKAGE} are	the exception, in that they return
       strings,	rather than references.	 These return the package and name of
       the typeglob itself, rather than	one that has been assigned to it.  So,
       after "*foo=*Foo::bar", *foo will become	"*Foo::bar" when used as a
       string, but *foo{PACKAGE} and *foo{NAME}	will continue to produce
       "main" and "foo", respectively.

       *foo{IO}	is an alternative to the *HANDLE mechanism given in "Typeglobs
       and Filehandles"	in perldata for	passing	filehandles into or out	of
       subroutines, or storing into larger data	structures.  Its disadvantage
       is that it won't	create a new filehandle	for you.  Its advantage	is
       that you	have less risk of clobbering more than you want	to with	a
       typeglob	assignment.  (It still conflates file and directory handles,
       though.)	 However, if you assign	the incoming value to a	scalar instead
       of a typeglob as	we do in the examples below, there's no	risk of	that
       happening.

	   splutter(*STDOUT);	       # pass the whole	glob
	   splutter(*STDOUT{IO});      # pass both file	and dir	handles

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

	   $rec	= get_rec(*STDIN);     # pass the whole	glob
	   $rec	= get_rec(*STDIN{IO}); # pass both file	and dir	handles

	   sub get_rec {
	       my $fh =	shift;
	       return scalar <$fh>;
	   }

   Using References
       That's it for creating references.  By now you're probably dying	to
       know how	to use references to get back to your long-lost	data.  There
       are several basic methods.

       Simple Scalar

       Anywhere	you'd put an identifier	(or chain of identifiers) as part of a
       variable	or subroutine name, you	can replace the	identifier with	a
       simple scalar variable containing a reference of	the correct type:

	   $bar	= $$scalarref;
	   push(@$arrayref, $filename);
	   $$arrayref[0] = "January";
	   $$hashref{"KEY"} = "VALUE";
	   &$coderef(1,2,3);
	   print $globref "output\n";

       It's important to understand that we are	specifically not dereferencing
       $arrayref[0] or $hashref{"KEY"} there.  The dereference of the scalar
       variable	happens	before it does any key lookups.	 Anything more
       complicated than	a simple scalar	variable must use methods 2 or 3
       below.  However,	a "simple scalar" includes an identifier that itself
       uses method 1 recursively.  Therefore, the following prints "howdy".

	   $refrefref =	\\\"howdy";
	   print $$$$refrefref;

       Block

       Anywhere	you'd put an identifier	(or chain of identifiers) as part of a
       variable	or subroutine name, you	can replace the	identifier with	a
       BLOCK returning a reference of the correct type.	 In other words, the
       previous	examples could be written like this:

	   $bar	= ${$scalarref};
	   push(@{$arrayref}, $filename);
	   ${$arrayref}[0] = "January";
	   ${$hashref}{"KEY"} =	"VALUE";
	   &{$coderef}(1,2,3);
	   $globref->print("output\n");	 # iff IO::Handle is loaded

       Admittedly, it's	a little silly to use the curlies in this case,	but
       the BLOCK can contain any arbitrary expression, in particular,
       subscripted expressions:

	   &{ $dispatch{$index}	}(1,2,3);      # call correct routine

       Because of being	able to	omit the curlies for the simple	case of	$$x,
       people often make the mistake of	viewing	the dereferencing symbols as
       proper operators, and wonder about their	precedence.  If	they were,
       though, you could use parentheses instead of braces.  That's not	the
       case.  Consider the difference below; case 0 is a short-hand version of
       case 1, not case	2:

	   $$hashref{"KEY"}   =	"VALUE";       # CASE 0
	   ${$hashref}{"KEY"} =	"VALUE";       # CASE 1
	   ${$hashref{"KEY"}} =	"VALUE";       # CASE 2
	   ${$hashref->{"KEY"}}	= "VALUE";     # CASE 3

       Case 2 is also deceptive	in that	you're accessing a variable called
       %hashref, not dereferencing through $hashref to the hash	it's
       presumably referencing.	That would be case 3.

       Arrow Notation

       Subroutine calls	and lookups of individual array	elements arise often
       enough that it gets cumbersome to use method 2.	As a form of syntactic
       sugar, the examples for method 2	may be written:

	   $arrayref->[0] = "January";	 # Array element
	   $hashref->{"KEY"} = "VALUE";	 # Hash	element
	   $coderef->(1,2,3);		 # Subroutine call

       The left	side of	the arrow can be any expression	returning a reference,
       including a previous dereference.  Note that $array[$x] is not the same
       thing as	"$array->[$x]" here:

	   $array[$x]->{"foo"}->[0] = "January";

       This is one of the cases	we mentioned earlier in	which references could
       spring into existence when in an	lvalue context.	 Before	this
       statement, $array[$x] may have been undefined.  If so, it's
       automatically defined with a hash reference so that we can look up
       "{"foo"}" in it.	 Likewise "$array[$x]->{"foo"}"	will automatically get
       defined with an array reference so that we can look up "[0]" in it.
       This process is called autovivification.

       One more	thing here.  The arrow is optional between brackets
       subscripts, so you can shrink the above down to

	   $array[$x]{"foo"}[0]	= "January";

       Which, in the degenerate	case of	using only ordinary arrays, gives you
       multidimensional	arrays just like C's:

	   $score[$x][$y][$z] += 42;

       Well, okay, not entirely	like C's arrays, actually.  C doesn't know how
       to grow its arrays on demand.  Perl does.

       Objects

       If a reference happens to be a reference	to an object, then there are
       probably	methods	to access the things referred to, and you should
       probably	stick to those methods unless you're in	the class package that
       defines the object's methods.  In other words, be nice, and don't
       violate the object's encapsulation without a very good reason.  Perl
       does not	enforce	encapsulation.	We are not totalitarians here.	We do
       expect some basic civility though.

       Miscellaneous Usage

       Using a string or number	as a reference produces	a symbolic reference,
       as explained above.  Using a reference as a number produces an integer
       representing its	storage	location in memory.  The only useful thing to
       be done with this is to compare two references numerically to see
       whether they refer to the same location.

	   if ($ref1 ==	$ref2) {  # cheap numeric compare of references
	       print "refs 1 and 2 refer to the	same thing\n";
	   }

       Using a reference as a string produces both its referent's type,
       including any package blessing as described in perlobj, as well as the
       numeric address expressed in hex.  The ref() operator returns just the
       type of thing the reference is pointing to, without the address.	 See
       "ref" in	perlfunc for details and examples of its use.

       The bless() operator may	be used	to associate the object	a reference
       points to with a	package	functioning as an object class.	 See perlobj.

       A typeglob may be dereferenced the same way a reference can, because
       the dereference syntax always indicates the type	of reference desired.
       So "${*foo}" and	"${\$foo}" both	indicate the same scalar variable.

       Here's a	trick for interpolating	a subroutine call into a string:

	   print "My sub returned @{[mysub(1,2,3)]} that time.\n";

       The way it works	is that	when the "@{...}" is seen in the double-quoted
       string, it's evaluated as a block.  The block creates a reference to an
       anonymous array containing the results of the call to "mysub(1,2,3)".
       So the whole block returns a reference to an array, which is then
       dereferenced by "@{...}"	and stuck into the double-quoted string. This
       chicanery is also useful	for arbitrary expressions:

	   print "That yields @{[$n + 5]} widgets\n";

       Similarly, an expression	that returns a reference to a scalar can be
       dereferenced via	"${...}". Thus,	the above expression may be written
       as:

	   print "That yields ${\($n + 5)} widgets\n";

   Circular References
       It is possible to create	a "circular reference" in Perl,	which can lead
       to memory leaks.	A circular reference occurs when two references
       contain a reference to each other, like this:

	   my $foo = {};
	   my $bar = { foo => $foo };
	   $foo->{bar} = $bar;

       You can also create a circular reference	with a single variable:

	   my $foo;
	   $foo	= \$foo;

       In this case, the reference count for the variables will	never reach 0,
       and the references will never be	garbage-collected. This	can lead to
       memory leaks.

       Because objects in Perl are implemented as references, it's possible to
       have circular references	with objects as	well. Imagine a	TreeNode class
       where each node references its parent and child nodes. Any node with a
       parent will be part of a	circular reference.

       You can break circular references by creating a "weak reference". A
       weak reference does not increment the reference count for a variable,
       which means that	the object can go out of scope and be destroyed. You
       can weaken a reference with the "weaken"	function exported by the
       Scalar::Util module, or available as "builtin::weaken" directly in Perl
       version 5.35.7 or later.

       Here's how we can make the first	example	safer:

	   use Scalar::Util 'weaken';

	   my $foo = {};
	   my $bar = { foo => $foo };
	   $foo->{bar} = $bar;

	   weaken $foo->{bar};

       The reference from $foo to $bar has been	weakened. When the $bar
       variable	goes out of scope, it will be garbage-collected. The next time
       you look	at the value of	the "$foo->{bar}" key, it will be "undef".

       This action at a	distance can be	confusing, so you should be careful
       with your use of	weaken.	You should weaken the reference	in the
       variable	that will go out of scope first. That way, the longer-lived
       variable	will contain the expected reference until it goes out of
       scope.

   Symbolic references
       We said that references spring into existence as	necessary if they are
       undefined, but we didn't	say what happens if a value used as a
       reference is already defined, but isn't a hard reference.  If you use
       it as a reference, it'll	be treated as a	symbolic reference.  That is,
       the value of the	scalar is taken	to be the name of a variable, rather
       than a direct link to a (possibly) anonymous value.

       People frequently expect	it to work like	this.  So it does.

	   $name = "foo";
	   $$name = 1;		       # Sets $foo
	   ${$name} = 2;	       # Sets $foo
	   ${$name x 2}	= 3;	       # Sets $foofoo
	   $name->[0] =	4;	       # Sets $foo[0]
	   @$name = ();		       # Clears	@foo
	   &$name();		       # Calls &foo()
	   $pack = "THAT";
	   ${"${pack}::$name"} = 5;    # Sets $THAT::foo without eval

       This is powerful, and slightly dangerous, in that it's possible to
       intend (with the	utmost sincerity) to use a hard	reference, and
       accidentally use	a symbolic reference instead.  To protect against
       that, you can say

	   use strict 'refs';

       and then	only hard references will be allowed for the rest of the
       enclosing block.	 An inner block	may countermand	that with

	   no strict 'refs';

       Only package variables (globals,	even if	localized) are visible to
       symbolic	references.  Lexical variables (declared with my()) aren't in
       a symbol	table, and thus	are invisible to this mechanism.  For example:

	   local $value	= 10;
	   $ref	= "value";
	   {
	       my $value = 20;
	       print $$ref;
	   }

       This will still print 10, not 20.  Remember that	local()	affects
       package variables, which	are all	"global" to the	package.

   Not-so-symbolic references
       Brackets	around a symbolic reference can	simply serve to	isolate	an
       identifier or variable name from	the rest of an expression, just	as
       they always have	within a string.  For example,

	   $push = "pop	on ";
	   print "${push}over";

       has always meant	to print "pop on over",	even though push is a reserved
       word.  This is generalized to work the same without the enclosing
       double quotes, so that

	   print ${push} . "over";

       and even

	   print ${ push } . "over";

       will have the same effect.  This	construct is not considered to be a
       symbolic	reference when you're using strict refs:

	   use strict 'refs';
	   ${ bareword };      # Okay, means $bareword.
	   ${ "bareword" };    # Error,	symbolic reference.

       Similarly, because of all the subscripting that is done using single
       words, the same rule applies to any bareword that is used for
       subscripting a hash.  So	now, instead of	writing

	   $hash{ "aaa"	}{ "bbb" }{ "ccc" }

       you can write just

	   $hash{ aaa }{ bbb }{	ccc }

       and not worry about whether the subscripts are reserved words.  In the
       rare event that you do wish to do something like

	   $hash{ shift	}

       you can force interpretation as a reserved word by adding anything that
       makes it	more than a bareword:

	   $hash{ shift() }
	   $hash{ +shift }
	   $hash{ shift	@_ }

       The "use	warnings" pragma or the	-w switch will warn you	if it
       interprets a reserved word as a string.	But it will no longer warn you
       about using lowercase words, because the	string is effectively quoted.

   Pseudo-hashes: Using	an array as a hash
       Pseudo-hashes have been removed from Perl.  The 'fields'	pragma remains
       available.

   Function Templates
       As explained above, an anonymous	function with access to	the lexical
       variables visible when that function was	compiled, creates a closure.
       It retains access to those variables even though	it doesn't get run
       until later, such as in a signal	handler	or a Tk	callback.

       Using a closure as a function template allows us	to generate many
       functions that act similarly.  Suppose you wanted functions named after
       the colors that generated HTML font changes for the various colors:

	   print "Be ",	red("careful"),	"with that ", green("light");

       The red() and green() functions would be	similar.  To create these,
       we'll assign a closure to a typeglob of the name	of the function	we're
       trying to build.

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

       Now all those different functions appear	to exist independently.	 You
       can call	red(), RED(), blue(), BLUE(), green(), etc.  This technique
       saves on	both compile time and memory use, and is less error-prone as
       well, since syntax checks happen	at compile time.  It's critical	that
       any variables in	the anonymous subroutine be lexicals in	order to
       create a	proper closure.	 That's	the reasons for	the "my" on the	loop
       iteration variable.

       This is one of the only places where giving a prototype to a closure
       makes much sense.  If you wanted	to impose scalar context on the
       arguments of these functions (probably not a wise idea for this
       particular example), you	could have written it this way instead:

	   *$name = sub	($) { "<FONT COLOR='$name'>$_[0]</FONT>" };

       However,	since prototype	checking happens at compile time, the
       assignment above	happens	too late to be of much use.  You could address
       this by putting the whole loop of assignments within a BEGIN block,
       forcing it to occur during compilation.

       Access to lexicals that change over time--like those in the "for" loop
       above, basically	aliases	to elements from the surrounding lexical
       scopes--	only works with	anonymous subs,	not with named subroutines.
       Generally said, named subroutines do not	nest properly and should only
       be declared in the main package scope.

       This is because named subroutines are created at	compile	time so	their
       lexical variables get assigned to the parent lexicals from the first
       execution of the	parent block. If a parent scope	is entered a second
       time, its lexicals are created again, while the nested subs still
       reference the old ones.

       Anonymous subroutines get to capture each time you execute the "sub"
       operator, as they are created on	the fly. If you	are accustomed to
       using nested subroutines	in other programming languages with their own
       private variables, you'll have to work at it a bit in Perl.  The
       intuitive coding	of this	type of	thing incurs mysterious	warnings about
       "will not stay shared" due to the reasons explained above.  For
       example,	this won't work:

	   sub outer {
	       my $x = $_[0] + 35;
	       sub inner { return $x * 19 }   #	WRONG
	       return $x + inner();
	   }

       A work-around is	the following:

	   sub outer {
	       my $x = $_[0] + 35;
	       local *inner = sub { return $x *	19 };
	       return $x + inner();
	   }

       Now inner() can only be called from within outer(), because of the
       temporary assignments of	the anonymous subroutine. But when it does, it
       has normal access to the	lexical	variable $x from the scope of outer()
       at the time outer is invoked.

       This has	the interesting	effect of creating a function local to another
       function, something not normally	supported in Perl.

   Postfix Dereference Syntax
       Beginning in v5.20.0, a postfix syntax for using	references is
       available.  It behaves as described in "Using References", but instead
       of a prefixed sigil, a postfixed	sigil-and-star is used.

       For example:

	   $r =	\@a;
	   @b =	$r->@*;	# equivalent to	@$r or @{ $r }

	   $r =	[ 1, [ 2, 3 ], 4 ];
	   $r->[1]->@*;	 # equivalent to @{ $r->[1] }

       In Perl 5.20 and	5.22, this syntax must be enabled with "use feature
       'postderef'". As	of Perl	5.24, no feature declarations are required to
       make it available.

       Postfix dereference should work in all circumstances where block
       (circumfix) dereference worked, and should be entirely equivalent.
       This syntax allows dereferencing	to be written and read entirely	left-
       to-right.  The following	equivalencies are defined:

	 $sref->$*;  # same as	${ $sref }
	 $aref->@*;  # same as	@{ $aref }
	 $aref->$#*; # same as $#{ $aref }
	 $href->%*;  # same as	%{ $href }
	 $cref->&*;  # same as	&{ $cref }
	 $gref->**;  # same as	*{ $gref }

       Note especially that "$cref->&*"	is not equivalent to "$cref->()", and
       can serve different purposes.

       Glob elements can be extracted through the postfix dereferencing
       feature:

	 $gref->*{SCALAR}; # same as *{	$gref }{SCALAR}

       Postfix array and scalar	dereferencing can be used in interpolating
       strings (double quotes or the "qq" operator), but only if the
       "postderef_qq" feature is enabled.

   Postfix Reference Slicing
       Value slices of arrays and hashes may also be taken with	postfix
       dereferencing notation, with the	following equivalencies:

	 $aref->@[ ... ];  # same as @$aref[ ... ]
	 $href->@{ ... };  # same as @$href{ ... }

       Postfix key/value pair slicing, added in	5.20.0 and documented in the
       Key/Value Hash Slices section of	perldata, also behaves as expected:

	 $aref->%[ ... ];  # same as %$aref[ ... ]
	 $href->%{ ... };  # same as %$href{ ... }

       As with postfix array, postfix value slice dereferencing	can be used in
       interpolating strings (double quotes or the "qq"	operator), but only if
       the "postderef_qq" feature is enabled.

   Assigning to	References
       Beginning in v5.22.0, the referencing operator can be assigned to.  It
       performs	an aliasing operation, so that the variable name referenced on
       the left-hand side becomes an alias for the thing referenced on the
       right-hand side:

	   \$a = \$b; #	$a and $b now point to the same	scalar
	   \&foo = \&bar; # foo() now means bar()

       This syntax must	be enabled with	"use feature 'refaliasing'".  It is
       experimental, and will warn by default unless "no warnings
       'experimental::refaliasing'" is in effect.

       These forms may be assigned to, and cause the right-hand	side to	be
       evaluated in scalar context:

	   \$scalar
	   \@array
	   \%hash
	   \&sub
	   \my $scalar
	   \my @array
	   \my %hash
	   \state $scalar # or @array, etc.
	   \our	$scalar	  # etc.
	   \local $scalar # etc.
	   \local our $scalar #	etc.
	   \$some_array[$index]
	   \$some_hash{$key}
	   \local $some_array[$index]
	   \local $some_hash{$key}
	   condition ? \$this :	\$that[0] # etc.

       Slicing operations and parentheses cause	the right-hand side to be
       evaluated in list context:

	   \@array[5..7]
	   (\@array[5..7])
	   \(@array[5..7])
	   \@hash{'foo','bar'}
	   (\@hash{'foo','bar'})
	   \(@hash{'foo','bar'})
	   (\$scalar)
	   \($scalar)
	   \(my	$scalar)
	   \my($scalar)
	   (\@array)
	   (\%hash)
	   (\&sub)
	   \(&sub)
	   \($foo, @bar, %baz)
	   (\$foo, \@bar, \%baz)

       Each element on the right-hand side must	be a reference to a datum of
       the right type.	Parentheses immediately	surrounding an array (and
       possibly	also "my"/"state"/"our"/"local") will make each	element	of the
       array an	alias to the corresponding scalar referenced on	the right-hand
       side:

	   \(@a) = \(@b); # @a and @b now have the same	elements
	   \my(@a) = \(@b); # likewise
	   \(my	@a) = \(@b); # likewise
	   push	@a, 3; # but now @a has	an extra element that @b lacks
	   \(@a) = (\$a, \$b, \$c); # @a now contains $a, $b, and $c

       Combining that form with	"local"	and putting parentheses	immediately
       around a	hash are forbidden (because it is not clear what they should
       do):

	   \local(@array) = foo(); # WRONG
	   \(%hash)	  = bar(); # WRONG

       Assignment to references	and non-references may be combined in lists
       and conditional ternary expressions, as long as the values on the
       right-hand side are the right type for each element on the left,	though
       this may	make for obfuscated code:

	   (my $tom, \my $dick,	\my @harry) = (\1, \2, [1..3]);
	   # $tom is now \1
	   # $dick is now 2 (read-only)
	   # @harry is (1,2,3)

	   my $type = ref $thingy;
	   ($type ? $type eq 'ARRAY' ? \@foo : \$bar : $baz) = $thingy;

       The "foreach" loop can also take	a reference constructor	for its	loop
       variable, though	the syntax is limited to one of	the following, with an
       optional	"my", "state", or "our"	after the backslash:

	   \$s
	   \@a
	   \%h
	   \&c

       No parentheses are permitted.  This feature is particularly useful for
       arrays-of-arrays, or arrays-of-hashes:

	   foreach \my @a (@array_of_arrays) {
	       frobnicate($a[0], $a[-1]);
	   }

	   foreach \my %h (@array_of_hashes) {
	       $h{gelastic}++ if $h{type} eq 'funny';
	   }

       CAVEAT: Aliasing	does not work correctly	with closures.	If you try to
       alias lexical variables from an inner subroutine	or "eval", the
       aliasing	will only be visible within that inner sub, and	will not
       affect the outer	subroutine where the variables are declared.  This
       bizarre behavior	is subject to change.

   Declaring a Reference to a Variable
       Beginning in v5.26.0, the referencing operator can come after "my",
       "state",	"our", or "local".  This syntax	must be	enabled	with "use
       feature 'declared_refs'".  It is	experimental, and will warn by default
       unless "no warnings 'experimental::refaliasing'"	is in effect.

       This feature makes these:

	   my \$x;
	   our \$y;

       equivalent to:

	   \my $x;
	   \our	$x;

       It is intended mainly for use in	assignments to references (see
       "Assigning to References", above).  It also allows the backslash	to be
       used on just some items in a list of declared variables:

	   my ($foo, \@bar, \%baz); # equivalent to:  my $foo, \my(@bar, %baz);

WARNING: Don't use references as hash keys
       You may not (usefully) use a reference as the key to a hash.  It	will
       be converted into a string:

	   $x{ \$a } = $a;

       If you try to dereference the key, it won't do a	hard dereference, and
       you won't accomplish what you're	attempting.  You might want to do
       something more like

	   $r =	\@a;
	   $x{ $r } = $r;

       And then	at least you can use the values(), which will be real refs,
       instead of the keys(), which won't.

       The standard Tie::RefHash module	provides a convenient workaround to
       this.

SEE ALSO
       Besides the obvious documents, source code can be instructive.  Some
       pathological examples of	the use	of references can be found in the
       t/op/ref.t regression test in the Perl source directory.

       See also	perldsc	and perllol for	how to use references to create
       complex data structures,	and perlootut and perlobj for how to use them
       to create objects.

perl v5.35.11			  2022-03-27			    PERLREF(1)

NAME | NOTE | DESCRIPTION | WARNING: Don't use references as hash keys | SEE ALSO

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