You are viewing the version of this documentation from Perl 5.35.9. This is a development version of Perl.



Scalar::Util - A selection of general-utility scalar subroutines


use Scalar::Util qw(blessed dualvar isdual readonly refaddr reftype
                    tainted weaken isweak isvstring looks_like_number
                    # and other useful utils appearing below


Scalar::Util contains a selection of subroutines that people have expressed would be nice to have in the perl core, but the usage would not really be high enough to warrant the use of a keyword, and the size would be so small that being individual extensions would be wasteful.

By default Scalar::Util does not export any subroutines.


The following functions all perform some useful activity on reference values.


my $pkg = blessed( $ref );

If $ref is a blessed reference, the name of the package that it is blessed into is returned. Otherwise undef is returned.

$scalar = "foo";
$class  = blessed $scalar;           # undef

$ref    = [];
$class  = blessed $ref;              # undef

$obj    = bless [], "Foo";
$class  = blessed $obj;              # "Foo"

Take care when using this function simply as a truth test (such as in if(blessed $ref)...) because the package name "0" is defined yet false.


my $addr = refaddr( $ref );

If $ref is reference, the internal memory address of the referenced value is returned as a plain integer. Otherwise undef is returned.

$addr = refaddr "string";           # undef
$addr = refaddr \$var;              # eg 12345678
$addr = refaddr [];                 # eg 23456784

$obj  = bless {}, "Foo";
$addr = refaddr $obj;               # eg 88123488


my $type = reftype( $ref );

If $ref is a reference, the basic Perl type of the variable referenced is returned as a plain string (such as ARRAY or HASH). Otherwise undef is returned.

$type = reftype "string";           # undef
$type = reftype \$var;              # SCALAR
$type = reftype [];                 # ARRAY

$obj  = bless {}, "Foo";
$type = reftype $obj;               # HASH

Note that for internal reasons, all precompiled regexps (qr/.../) are blessed references; thus ref() returns the package name string "Regexp" on these but reftype() will return the underlying C structure type of "REGEXP" in all capitals.


weaken( $ref );

The lvalue $ref will be turned into a weak reference. This means that it will not hold a reference count on the object it references. Also, when the reference count on that object reaches zero, the reference will be set to undef. This function mutates the lvalue passed as its argument and returns no value.

This is useful for keeping copies of references, but you don't want to prevent the object being DESTROY-ed at its usual time.

  my $var;
  $ref = \$var;
  weaken($ref);                     # Make $ref a weak reference
# $ref is now undef

Note that if you take a copy of a scalar with a weakened reference, the copy will be a strong reference.

my $var;
my $foo = \$var;
weaken($foo);                       # Make $foo a weak reference
my $bar = $foo;                     # $bar is now a strong reference

This may be less obvious in other situations, such as grep(), for instance when grepping through a list of weakened references to objects that may have been destroyed already:

@object = grep { defined } @object;

This will indeed remove all references to destroyed objects, but the remaining references to objects will be strong, causing the remaining objects to never be destroyed because there is now always a strong reference to them in the @object array.


unweaken( $ref );

Since version 1.36.

The lvalue REF will be turned from a weak reference back into a normal (strong) reference again. This function mutates the lvalue passed as its argument and returns no value. This undoes the action performed by "weaken".

This function is slightly neater and more convenient than the otherwise-equivalent code

my $tmp = $REF;
undef $REF;
$REF = $tmp;

(because in particular, simply assigning a weak reference back to itself does not work to unweaken it; $REF = $REF does not work).


my $weak = isweak( $ref );

Returns true if $ref is a weak reference.

$ref  = \$foo;
$weak = isweak($ref);               # false
$weak = isweak($ref);               # true

NOTE: Copying a weak reference creates a normal, strong, reference.

$copy = $ref;
$weak = isweak($copy);              # false



my $var = dualvar( $num, $string );

Returns a scalar that has the value $num in a numeric context and the value $string in a string context.

$foo = dualvar 10, "Hello";
$num = $foo + 2;                    # 12
$str = $foo . " world";             # Hello world


my $dual = isdual( $var );

Since version 1.26.

If $var is a scalar that has both numeric and string values, the result is true.

$foo = dualvar 86, "Nix";
$dual = isdual($foo);               # true

Note that a scalar can be made to have both string and numeric content through standard operations:

$foo = "10";
$dual = isdual($foo);               # false
$bar = $foo + 0;
$dual = isdual($foo);               # true

The $! variable is commonly dual-valued, though it is also magical in other ways:

$! = 1;
$dual = isdual($!);                 # true
print("$!\n");                      # "Operation not permitted"

CAUTION: This function is not as useful as it may seem. Dualvars are not a distinct concept in Perl, but a standard internal construct of all scalar values. Almost any value could be considered as a dualvar by this function through the course of normal operations.


my $vstring = isvstring( $var );

If $var is a scalar which was coded as a vstring, the result is true.

$vs   = v49.46.48;
$fmt  = isvstring($vs) ? "%vd" : "%s"; #true


my $isnum = looks_like_number( $var );

Returns true if perl thinks $var is a number. See "looks_like_number" in perlapi.


my $fh = openhandle( $fh );

Returns $fh itself, if $fh may be used as a filehandle and is open, or if it is a tied handle. Otherwise undef is returned.

$fh = openhandle(*STDIN);           # \*STDIN
$fh = openhandle(\*STDIN);          # \*STDIN
$fh = openhandle(*NOTOPEN);         # undef
$fh = openhandle("scalar");         # undef


my $ro = readonly( $var );

Returns true if $var is readonly.

sub foo { readonly($_[0]) }

$readonly = foo($bar);              # false
$readonly = foo(0);                 # true


my $code = set_prototype( $code, $prototype );

Sets the prototype of the function given by the $code reference, or deletes it if $prototype is undef. Returns the $code reference itself.

set_prototype \&foo, '$$';


my $t = tainted( $var );

Return true if $var is tainted.

$taint = tainted("constant");       # false
$taint = tainted($ENV{PWD});        # true if running under -T


Module use may give one of the following errors during import.

Vstrings are not implemented in this version of perl

The version of perl that you are using does not implement Vstrings, to use "isvstring" you will need to use a newer release of perl.


There is a bug in perl5.6.0 with UV's that are >= 1<<31. This will show up as tests 8 and 9 of dualvar.t failing




Copyright (c) 1997-2007 Graham Barr <>. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

Additionally "weaken" and "isweak" which are

Copyright (c) 1999 Tuomas J. Lukka <>. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as perl itself.

Copyright (C) 2004, 2008 Matthijs van Duin. All rights reserved. Copyright (C) 2014 cPanel Inc. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.