my $ar = [];
push @{$ar}, $ar;
# $ar can only be destroyed manually.

##</code><code>##

use Scalar::Util qw( weaken is_weak );

my $ar = [];
push @{ $ar }, weaken $ar;
# $ar will be destroyed normally.

if ( is_weak $ar->[0] ) {
    print "First list item is weak\n";
}
my $copy = $ar->[0];
if ( ! is_weak $copy ) {
    print "copy is not weak\n";
}