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 );

##</code><code>##

Cycle (1):
                   $Top::A->{'bottom'} => \%Bottom::B
                   $Bottom::B->{'top'} => \%Top::A

##</code><code>##

local $| = 1;
for( 1...1_000_000 )
{
  print "\r$_/1000000";
  my $top = Top->new();
}

##</code><code>##

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!";