#!/usr/local/bin/perl
use strict;
use warnings;
use Scalar::Util qw/weaken/;
{
    package Dummy;
    sub new { bless $_[1], $_[0] }
    sub DESTROY { warn "($_[0])" }
}
{
    my $mom = Dummy->new( {} );
    my $son = Dummy->new( {} );
    $mom->{son} = $son;
    weaken $mom->{son};
    $son->{mom} = $mom;
    weaken $son->{mom};
}
warn "Goodbye, world!";