my $a = [];
my $b = [ $a ];
push @$a, $b;
# Now $b contains a reference to $a and 
# $a contains a reference to $b.

##</code><code>##

sub marry
{
  my ($husband, $wife) = @_;
  $husband->set_spouse($wife);
  $wife->set_spouse($husband);
  $wife->parent[0]->pay_for_party;
}