package Top;
use Scalar::Util 'weaken';

sub new {
  my $class = shift;
  my $s = bless { }, $class;
  weaken($s);
  $s->{bottom} = Bottom->new( $s );
  return $s;
}

#=====================================

package Bottom;
use Scalar::Util 'weaken';

sub new {
  my ($class, $top) = @_;
  my $s = bless { top => $top }, $class;
  weaken($s);
  return $s;
}

#=====================================

package main;

use strict;
use warnings 'all';

my $top = Top->new();
my $bottom = $top->{bottom} or die "NO BOTTOM!";