use strict;
use Storable qw(freeze thaw);
use Scalar::Util qw(weaken isweak);

# make a circular reference, then weaken it
my $a={};
$a->{a}=\$a;
weaken($a->{a});

# looks like we're ok
isweak($a->{a}) || die;

# serilize it
my $b=freeze($a);

# and unpack it
my $c=thaw($b);

# check if the weak reference survived the freeze
isweak($c->{a}) || die "nope!";