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Clone-0.50_01
⭐ Starred 7 / Clone

NAME

Clone - recursively copy Perl datatypes

Build Status CPAN version

SYNOPSIS

use Clone 'clone';

my $data = {
   set => [ 1 .. 50 ],
   foo => {
       answer => 42,
       object => SomeObject->new,
   },
};

my $cloned_data = clone($data);

$cloned_data->{foo}{answer} = 1;
print $cloned_data->{foo}{answer};  # '1'
print $data->{foo}{answer};         # '42'

You can also add it to your class:

package Foo;
use parent 'Clone';
sub new { bless {}, shift }

package main;

my $obj = Foo->new;
my $copy = $obj->clone;

DESCRIPTION

This module provides a clone() method which makes recursive copies of nested hash, array, scalar and reference types, including tied variables and objects.

clone() takes a scalar argument and duplicates it. To duplicate lists, arrays or hashes, pass them in by reference, e.g.

my $copy = clone (\@array);

# or

my %copy = %{ clone (\%hash) };

EXAMPLES

Cloning Blessed Objects

package Person;
sub new {
    my ($class, $name) = @_;
    bless { name => $name, friends => [] }, $class;
}

package main;
use Clone 'clone';

my $person = Person->new('Alice');
my $clone = clone($person);

# $clone is a separate object with the same data
push @{$person->{friends}}, 'Bob';
print scalar @{$clone->{friends}};  # 0

Handling Circular References

Clone properly handles circular references, preventing infinite loops:

my $a = { name => 'A' };
my $b = { name => 'B', ref => $a };
$a->{ref} = $b;  # circular reference

my $clone = clone($a);
# Circular structure is preserved in the clone

Cloning Weakened References

use Scalar::Util 'weaken';

my $obj = { data => 'important' };
my $container = { strong => $obj, weak => $obj };
weaken($container->{weak});

my $clone = clone($container);
# Both strong and weak references are preserved correctly

Cloning Tied Variables

use Tie::Hash;
tie my %hash, 'Tie::StdHash';
%hash = (a => 1, b => 2);

my $clone = clone(\%hash);
# The tied behavior is preserved in the clone

LIMITATIONS

  • Maximum Recursion Depth

    Clone uses a recursion depth counter to prevent stack overflow. The default limit is 4000 rdepth units on Linux/macOS and 2000 on Windows/Cygwin. Each nesting level consumes approximately 2 rdepth units, so the effective limits are roughly 2000 nesting levels on Linux/macOS and 1000 on Windows/Cygwin.

    When the limit is exceeded, Clone switches to an iterative fallback that preserves deep-copy semantics without stack overflow. This covers arrays, hashes, and all reference types (including deeply nested scalar references). The fallback drives nested containers through a heap-allocated work queue, so its C stack usage does not grow with nesting depth whatever the shape of the data.

    Non-clonable types (globs, code references, formats, IO handles) are always shared regardless of depth. Encountering one directly as a container element past the depth limit also emits a warning (one reached through a reference is shared silently, as at any depth). To silence it:

    $Clone::WARN = 0;

    You can override the depth limit by passing it as the second argument to clone():

    my $copy = clone($data, 8000);  # allow deeper recursion

  • Filehandles and IO Objects

    Filehandles and IO objects are not deep-copied. The clone shares the same underlying filehandle object as the original (reference count is incremented). For DBI database handles, Clone skips opaque XS magic to avoid dangling pointers, but the resulting clone should not be used as a database handle.

  • Code References

    Code references (subroutines) are cloned by reference, not by value. The cloned coderef points to the same subroutine as the original.

  • Thread Safety

    Clone is not explicitly thread-safe. Use appropriate synchronization when cloning data structures across threads.

PERFORMANCE

Clone is implemented in C using Perl's XS interface, making it very fast for most use cases.

  • When to use Clone

    Clone is optimized for speed and works best with:

    • Shallow to medium-depth structures (3 levels or fewer)

    • Data structures that need fast cloning in hot code paths

    • Structures containing blessed objects and tied variables

  • When to use Storable::dclone

    Storable's dclone() may be faster for:

    • Very deep structures (4+ levels)

    • When you need serialization features

Benchmarking your specific use case is recommended for performance-critical applications.

CAVEATS

  • Cloned objects are deep copies

    Changes to the clone do not affect the original, and vice versa. This includes nested references and objects.

  • Object internals

    While Clone handles most blessed objects correctly, objects with XS components or complex internal state may not clone as expected. Test thoroughly with your specific object types.

  • Memory usage

    Cloning large data structures creates a complete copy in memory. Ensure you have sufficient memory available.

SEE ALSO

Storable's dclone() is a flexible solution for cloning variables, albeit slower for average-sized data structures. Simple and naive benchmarks show that Clone is faster for data structures with 3 or fewer levels, while dclone() can be faster for structures 4 or more levels deep.

Other modules that may be of interest:

Clone::PP - Pure Perl implementation of Clone

Scalar::Util - For weaken() and other scalar utilities

Data::Dumper - For debugging and inspecting data structures

SUPPORT

COPYRIGHT

Copyright 2001-2026 Ray Finch. All Rights Reserved.

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

AUTHOR

Ray Finch <rdf@cpan.org>

Breno G. de Oliveira <garu@cpan.org>, Nicolas Rochelemagne <atoomic@cpan.org> and Florian Ragwitz <rafl@debian.org> perform routine maintenance releases since 2012.