my $dad= My::Parent->new( name => 'Robert' );
    {
        my $son= My::Child->new( $dad, name => 'Bobby' );
    }
    # No direct access to Bobby at this point.
    # That triggers garbage collection.
    # Bobby and Robert circularly reference each other.
    # Robert is still reachable from the outside.
    # Bobby does not get destroyed, because we can reach him indirectly:
    {
        my $son= $dad->GetSon();    # Got Bobby again
    }

    $dad= My::Parent->new( name => 'Walter' );
    # No direct access to Robert, now.  Garbage collection runs.
    # Robert and Bobby form a loop but both are unreferenced.
    # Both get destroyed.