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perlref (1)


perlref - Perl references and nested data structures


Please see following description for synopsis


Perl Programmers Reference Guide                       PERLREF(1)

     perlref - Perl references and nested data structures

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

     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 "Two-Phased
     Garbage Collection" in perlobj 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

     References are easy to use in Perl.  There is just one
     overriding principle: 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.

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  Making References
     References can be created in several ways.

     1.  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.

     2.  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).

     3.  A reference to an anonymous hash can be created using

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         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

     4.  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

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         "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

         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...


         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.

     5.  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

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         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

             $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)

     6.  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.

     7.  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

             $scalarref = *foo{SCALAR};
             $arrayref  = *ARGV{ARRAY};
             $hashref   = *ENV{HASH};
             $coderef   = *handler{CODE};
             $ioref     = *STDIN{IO};
             $globref   = *foo{GLOB};
             $formatref = *foo{FORMAT};

         All of these are self-explanatory except for *foo{IO}.
         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 deprecated as of 5.8.0.  If
         deprecation warnings are in effect, it will warn of its

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         *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{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.

     1.  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";
             print $globref "output\n";

         It's important to understand that we are specifically

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         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;

     2.  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";
             $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.

     3.  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:

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             $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

     4.  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.

     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.

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         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

         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";

  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.

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         $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() (as in Perl 4)
         $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

         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

  Not-so-symbolic references
     A new feature contributing to readability in perl version
     5.001 is that the brackets around a symbolic reference
     behave more like quotes, just as they always have within a
     string.  That is,

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

     has always meant to print "pop on over", even though push is
     a reserved word.  This has been generalized to work the same
     outside of quotes, so that

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         print ${push} . "over";

     and even

         print ${ push } . "over";

     will have the same effect.  (This would have been a syntax
     error in Perl 5.000, though Perl 4 allowed it in the
     spaceless form.)  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, we've applied the same rule to any
     bareword that is used for subscripting a hash.  So now,
     instead of writing

         $array{ "aaa" }{ "bbb" }{ "ccc" }

     you can write just

         $array{ aaa }{ bbb }{ ccc }

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

         $array{ shift }

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

         $array{ shift() }
         $array{ +shift }
         $array{ 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

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     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

     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

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     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

     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

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

     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;

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     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 attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     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 perltoot, perlobj, and
     perlbot for how to use them to create objects.

     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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