sub toggle_door {

  my ($door_ref) = @_;

  state $transition = { qw(Open Closed  Closed Open) };

  $$door_ref // die 'undefined door state';
  exists $transition->{$$door_ref}
    or die qq{unknown door state '$$door_ref'};

  return $$door_ref = $transition->{$$door_ref};

  }