# Parent
sub get_child_id {

  my $self=shift;
  my $child=shift;
  # Find the index of the child. Left as exercise for the reader (read: I'm too lazy :) ).
  return $child_index;
}

##</code><code>##

# Child
sub whereami {

  my $self=shift;
  my $parent=$self->{'parent'};
  my $id=$parent->get_child_id($self);

}

##</code><code>##

my $self = {
      parent => \$parent;
   };

##</code><code>##

my $self= { parent => $parent }; #$parent is a ref, you were storing a ref to $parent