You can always check to see if it's there:

#!/usr/bin/perl -l

use strict;
use warnings;
use List::Util::XS;

print $List::Util::XS::VERSION;
[download]

And here's a bundle to ensure that you have weakly things:

#!/usr/bin/perl

use strict;
use warnings;
use CPAN;

CPAN::Shell->install(qw(
Task::Weaken
Cache::Weak
Class::IntrospectionMethods
Class::WeakSingleton
Class::Observable
Clone::Closure
Convert::Scalar
Data::Decycle
Data::Dump::Streamer
Data::Structure::Util
Devel::Cycle
EO::WeakArray
FCGI::EV
Hash::NoRef
IO::Plumbing
Object::InsideOut
Memoize::Expire::ByInstance
Scalar::Util
Set::Object
Test::Memory::Cycle
Test::NoXS
Test::Weaken
Test::Weaken::ExtraBits
Tie::RefHash
Tie::RefHash::Weak
Tie::Util
Util::Any
WeakRef
WeakRef::Auto));
[download]