PERLREF(1)

💖 NAME 💖

perlref — Perl references and nested data structures. okay so this is the manual page for perlref, and bestie, it's literally all about Perl references and nested data structures. no cap. 💅

---

✨ NOTE ✨

This is complete documentation about all aspects of references. For a shorter, tutorial introduction to just the essential features, see perlreftut. it's giving beginner-friendly. 🫶

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🌸 DESCRIPTION 🌸

before release 5 of Perl, representing complex data structures was lowkey a struggle, because all references had to be symbolic—and even then it was hard to refer to a variable instead of a symbol table entry. 😭 but now? Perl not only makes it easier to use symbolic references to variables, it also lets you have hard references to any piece of data or code. any scalar can hold a hard reference. and 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. it's giving infinite nesting energy. 💅

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 like 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're 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's usually talking about a hard reference. ✨

references are easy to use in Perl. there's 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. periodt. 💅

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🎀 Making References 🎀

references can be created in several ways. let's get into it.

🦋 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 = (\$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 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)

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 requested, it is best to avoid indirect object syntax entirely. 🚩

🙈 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. 💀➡️💖

Okay so quick lil follow-up on the globs before we move on, because there's a gotcha:

• *foo{THING} hands you back undef if that particular THING hasn't been used yet — except for scalars. *foo{SCALAR} gives you a reference to an anonymous scalar if $foo hasn't been used yet. (Heads up bestie: this might change in a future release. 👀)
• *foo{NAME} and *foo{PACKAGE} are the exceptions — they return strings, not references. And they give you the package and name of the typeglob itself, not whatever's been assigned to it. So after *foo=*Foo::bar, *foo becomes "*Foo::bar" when you use it as a string, but *foo{PACKAGE} and *foo{NAME} keep on producing "main" and "foo", respectively. They simply do not care. 💅
• *foo{IO} is a whole alternative to the *HANDLE mechanism described in "Typeglobs and Filehandles" in perldata — it's for passing filehandles into or out of subroutines, or stashing them into bigger data structures. The downside? It won't create a new filehandle for you. 😭 The upside? Way less risk of clobbering more than you meant to with a typeglob assignment. (It does still conflate file and directory handles, though. 🙈) However — if you assign the incoming value to a scalar instead of a typeglob, like we do in the examples below, there's literally zero 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 🦋

Okay so that's creating references, done and dusted. By now you're probably like "bestie I'm dying, how do I get back to my long-lost data??" 😩 Totally fair. There are a few basic methods. Let's go.

💖 Simple Scalar 💖

Anywhere you'd put an identifier (or a chain of identifiers) as part of a variable or subroutine name, you can swap that identifier out for a simple scalar variable holding 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 is so important to get this: we are specifically not dereferencing $arrayref[0] or $hashref{"KEY"} there. The dereference of the scalar variable happens before any key lookups happen. Anything more complicated than a simple scalar variable has to use method 2 or method 3 below, no cap. 🫶

But also — a "simple scalar" includes an identifier that itself uses method 1 recursively. So this next one prints "howdy":

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

🌸 Block 🌸

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

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

Okay yes, admittedly the curlies are a lil silly in that exact case 😅 but the BLOCK can hold any arbitrary expression — in particular, subscripted expressions:

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

Because you can omit the curlies for the simple $$x case, people constantly make the mistake of thinking the dereferencing symbols are proper operators, and then they start wondering about their precedence. But if they were operators, you could use parentheses instead of braces. You can't. Not the vibe. 💀

So check out 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 extra deceptive too, because you're actually 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 come up often enough that method 2 gets genuinely cumbersome. So as a form of syntactic sugar, those method 2 examples may be written like this:

$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 those cases we mentioned earlier where references can just spring into existence when you're in an lvalue context. ✨ Before this statement, $array[$x] may have been undefined. If it was, it automatically gets defined with a hash reference so we can look up "{"foo"}" in it. Likewise, "$array[$x]->{"foo"}" automatically gets defined with an array reference so we can look up "[0]" in it. This whole process is called autovivification. Iconic behaviour, honestly. 👑

One more thing here: the arrow is optional between bracket subscripts, so you can shrink that whole thing 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 being referred to, and you should probably stick to those methods — unless you're inside 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. But we do expect some basic civility, bestie.

⭐ 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 you can do with that is 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 — and 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 how to use it.

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. Cute, right?

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: when the "@{...}" is seen inside the double-quoted string, it gets evaluated as a block. That 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 then gets 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 "${...}". So the above expression may also be written as:

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

---

😭 Circular References 😭

So it is possible to create a "circular reference" in Perl, and that can lead to memory leaks. A circular reference happens 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. Which leads to memory leaks. 😩

And because objects in Perl are implemented as references, you can get circular references with objects too. Picture a TreeNode class where each node references its parent and child nodes. Any node with a parent will be part of a circular reference. It's giving inescapable web.

You can break circular references by creating a "weak reference". A weak reference does not increment the reference count for a variable, which means 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 that 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 $bar variable goes out of scope, it will be garbage-collected. And the next time you look at the value of the "$foo->{bar}" key, it will be "undef". 🫶

This action at a distance can be confusing, so 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 references spring into existence as necessary if they're undefined, but we never said what happens if a value used as a reference is already defined but isn't a hard reference. Well: if you use it as a reference, it'll be treated as a symbolic reference. Meaning 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 also lowkey dangerous, in that it is absolutely possible to intend (with the utmost sincerity, bestie, like you meant it) to use a hard reference, and then accidentally use a symbolic reference instead 🙈 To protect against that whole situation, you can say:

use strict 'refs';

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

no strict 'refs';

Okay but here's the thing: only package variables (globals, even if they're localized) are visible to symbolic references. Lexical variables (the ones declared with my()) aren't in a symbol table, so they're literally invisible to this mechanism. Like, watch:

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 just be there to isolate an identifier or variable name from the rest of an expression — exactly like they always have inside 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 👑 And this got generalized to work the same way without the enclosing double quotes, so that:

print ${push} . "over";

and even:

print ${ push } . "over";

both have the same effect. And here's the slay part: this construct is not considered 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 gets done using single words — the same rule applies to any bareword used for subscripting a hash. So now, instead of writing:

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

you can just write:

$hash{ aaa }{ bbb }{ ccc }

and never worry about whether the subscripts are reserved words 🫶 In the rare event that you do wanna do something like:

$hash{ shift }

you can force interpretation as a reserved word by adding literally 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. (rip to a real one, but the fields pragma is still right there if you need it 🎀)

🎀 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, like in a signal handler or a Tk callback.

Using a closure as a function template lets us generate a whole bunch of functions that act similarly 🌈 Suppose you wanted functions named after 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 reason 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, which are 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, because they're created on the fly. If you're 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 get called, 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] }

$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 '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. 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 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:

\$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 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:

\(@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 even do):

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

Assignment to references and non-references can totally be mixed and matched in lists and conditional ternary expressions, bestie, as long as the right-hand side values are the right type for each left-hand element — though ngl this can make for some truly 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 like, for example

@b = ((\$l1, \$l2, \(@a)) = (\$r1, \$r2, $\r3, \$4));

is literally 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 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. Periodt. 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. so like, be careful bestie 💀

---

🎀 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. it's giving "why is this not working" energy fr fr 😭 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. slay, there's a fix 🫶

---

✨ 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. go build something iconic bestie 👑💖

---

and that's a wrap on PERLREF(1) from perl v5.42.3, dated 2026-08-03 🦋✨ literally the whole page, ate and left no crumbs, ty for reading my ted talk 🎀