package Top;

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

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

package Bottom;

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

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

package main;

use strict;
use warnings 'all';
use Devel::Cycle;

my $top = Top->new();
find_cycle( $top );