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 perlref-
tut.
DESCRIPTION
Before release 5 of Perl it was difficult to represent complex data struc-
tures, because all references had to be symbolic--and even then it was dif-
ficult 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, au-
tomatically 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 Refer-
ences" 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 sym-
bolic 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 sys-
tem: They are used to access an underlying object without concern for what
its (other) name is. When the word "reference" is used without an adjec-
tive, 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 princi-
ple: 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 refer-
ence 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 = (\$x, \@y, \%z);
@list = \($x, @y, %z); # 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 syn-
tax 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 real-
izes 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 ref-
erence 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" with-
out a subname:
$coderef = sub { print "Boink!\n" };
Note the semicolon. Except for the code inside not being immediately exe-
cuted, 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 no-
tion 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 lit-
tle bits of code to run later, such as callbacks. You can even do ob-
ject-oriented stuff with it, though Perl already provides a different mech-
anism 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 new-
print() 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 con-
tinue 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 start-
ing 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 bet-
ter 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)
This indirect object syntax is only available when "use feature "indirect""
is in effect, and that is not the case when "use v5.36" (or higher) is re-
quested, it is best to avoid indirect object syntax entirely.
Autovivification
References of the appropriate type can spring into existence if you deref-
erence 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 atten-
tion. It returns the IO handle, used for file handles ("open" in perl-
func), sockets ("socket" in perlfunc and "socketpair" in perlfunc), and di-
rectory 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 be-
tween v5.8 and v5.22, it will issue a deprecation warning, but this depre-
cation 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 fu-
ture 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", re-
spectively.
*foo{IO} is an alternative to the *HANDLE mechanism given in "Typeglobs and
Filehandles" in perldata for passing filehandles into or out of subrou-
tines, 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 as-
sign 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 sev-
eral 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";
$$hashref{"KEY"} = "VALUE";
&$coderef(1,2,3);
print $globref "output\n";
It's important to understand that we are specifically not dereferencing
$arrayref[0] or $hashref{"KEY"} there. The dereference of the scalar vari-
able happens before it does any key lookups. Anything more complicated
than a simple scalar variable must use methods 2 or 3 below. However, a
"simple scalar" includes an identifier that itself uses method 1 recur-
sively. Therefore, the following prints "howdy".
$refrefref = \\\"howdy";
print $$$$refrefref;
Block
Anywhere you'd put an identifier (or chain of identifiers) as part of a
variable or subroutine name, you can replace the identifier with a BLOCK
returning a reference of the correct type. In other words, the previous
examples could be written like this:
$bar = ${$scalarref};
push(@{$arrayref}, $filename);
${$arrayref}[0] = "January";
${$hashref}{"KEY"} = "VALUE";
&{$coderef}(1,2,3);
$globref->print("output\n"); # iff IO::Handle is loaded
Admittedly, it's a little silly to use the curlies in this case, but the
BLOCK can contain any arbitrary expression, in particular, subscripted ex-
pressions:
&{ $dispatch{$index} }(1,2,3); # call correct routine
Because of being able to omit the curlies for the simple case of $$x, peo-
ple 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, in-
cluding 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 "$ar-
ray[$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 mul-
tidimensional 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 prob-
ably 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 encapsula-
tion. 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 repre-
senting its storage location in memory. The only useful thing to be done
with this is to compare two references numerically to see whether they re-
fer to the same location.
if ($ref1 == $ref2) { # cheap numeric compare of references
print "refs 1 and 2 refer to the same thing\n";
}
Using a reference as a string produces both its referent's type, including
any package blessing as described in perlobj, as well as the numeric ad-
dress expressed in hex. The ref() operator returns just the type of thing
the reference is pointing to, without the address. See "ref" in perlfunc
for details and examples of its use.
The bless() operator may be used to associate the object a reference points
to with a package functioning as an object class. See perlobj.
A typeglob may be dereferenced the same way a reference can, because the
dereference syntax always indicates the type of reference desired. So
"${*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 deref-
erenced 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 par-
ent 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 con-
tain the expected reference until it goes out of scope.
Symbolic references
We said that references spring into existence as necessary if they are un-
defined, 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 (pos-
sibly) anonymous value.
People frequently expect it to work like this. So it does.
$name = "foo";
$$name = 1; # Sets $foo
${$name} = 2; # Sets $foo
${$name x 2} = 3; # Sets $foofoo
$name->[0] = 4; # Sets $foo[0]
@$name = (); # Clears @foo
&$name(); # Calls &foo()
$pack = "THAT";
${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
This is powerful, and slightly dangerous, in that it's possible to intend
(with the utmost sincerity) to use a hard reference, and accidentally use a
symbolic reference instead. To protect against that, you can say
use strict 'refs';
and then only hard references will be allowed for the rest of the enclosing
block. An inner block may countermand that with
no strict 'refs';
Only package variables (globals, even if localized) are visible to symbolic
references. Lexical variables (declared with my()) aren't in a symbol ta-
ble, and thus are invisible to this mechanism. For example:
local $value = 10;
$ref = "value";
{
my $value = 20;
print $$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 identi-
fier 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 sym-
bolic 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 low-
ercase 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 vari-
ables visible when that function was compiled, creates a closure. It re-
tains 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 $name} = sub { "@_" };
}
Now all those different functions appear to exist independently. You can
call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on
both compile time and memory use, and is less error-prone as well, since
syntax checks happen at compile time. It's critical that any variables in
the anonymous subroutine be lexicals in order to create a proper closure.
That's the reasons for the "my" on the loop iteration variable.
This is one of the only places where giving a prototype to a closure makes
much sense. If you wanted to impose scalar context on the arguments of
these functions (probably not a wise idea for this particular example), you
could have written it this way instead:
*$name = sub ($) { "$_[0]" };
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 lex-
ical 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 lexi-
cals are created again, while the nested subs still reference the old ones.
Anonymous subroutines get to capture each time you execute the "sub" opera-
tor, as they are created on the fly. If you are accustomed to using nested
subroutines in other programming languages with their own private vari-
ables, 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 tempo-
rary assignments of the anonymous subroutine. But when it does, it has nor-
mal 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] }
$aref->@* = (1, 2, 3); # same as @{ $aref } = (1, 2, 3)
In Perl 5.20 and 5.22, this syntax must be enabled with use feature 'post-
deref'. As of Perl 5.24, no feature declarations are required to make it
available.
Postfix dereference should work in all circumstances where block (circum-
fix) 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 *{ $gref }{SCALAR}
Postfix array and scalar dereferencing can be used in interpolating strings
(double quotes or the "qq" operator), but only if the "postderef_qq" fea-
ture is enabled. Interpolation of postfix array highest index access
("->$#*") is also supported when the "postderef_qq" feature is enabled.
Postfix Reference Slicing
Value slices of arrays and hashes may also be taken with postfix derefer-
encing 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 in-
terpolating 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 per-
forms 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:
\$x = \$y; # $x and $y now point to the same scalar
\&foo = \&bar; # foo() now means bar()
This syntax must be enabled with "use feature 'refaliasing'". It is exper-
imental, and will warn by default unless no warnings 'experimental::re-
faliasing' is in effect.
These forms may be assigned to, and cause the right-hand side to be evalu-
ated 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 evalu-
ated 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:
\(@x) = \(@y); # @x and @y now have the same elements
\my(@x) = \(@y); # likewise
\(my @x) = \(@y); # likewise
push @x, 3; # but now @x has an extra element that @y lacks
\(@x) = (\$x, \$y, \$z); # @x now contains $x, $y, and $z
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;
A reference assignment in list context returns a list of references to each
left-hand value. So for example
@b = ((\$l1, \$l2, \(@a)) = (\$r1, \$r2, $\r3, \$4));
is equivalent to
(\$l1, \$l2, \(@a)) = (\$r1, \$r2, $\r3, \$4);
@b = (\$l1, \$l2, \$a[0], \$a[1]);
The "foreach" loop can also take a reference constructor for its loop vari-
able, though the syntax is limited to one of the following, with an op-
tional "my", "state", or "our" after the backslash:
\$s
\@a
\%h
\&c
No parentheses are permitted. This feature is particularly useful for ar-
rays-of-arrays, or arrays-of-hashes:
foreach \my @a (@array_of_arrays) {
frobnicate($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 sub-
ject 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{ \$x } = $x;
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, in-
stead 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 patho-
logical 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 v5.44.0 2026-05-14 PERLREF(1)