package A;
sub new { 
  my $class = shift;
  my $this = bless {}, $class;
  $this->{-twin} = new B(-twin => $this);
  return $this;
}
package B;
sub new {
  my $class = shift;
  my $this = bless {@_}, $class;
  weaken $this->{-twin};
  return $this;
}
$obj = new A;
undef $obj;