sub toggle_door
  {
    my $door_ref = shift;
    if( $$door_ref eq 'Closed' )
    {
      $$door_ref = 'Open';
    } else {
      $$door_ref = 'Closed';
    }
    return $$door_ref; 
  }

##</code><code>##

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};

  }

##</code><code>##

sub toggle_door {

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

  $_[0] // die 'undefined door state';
  exists $transition->{$_[0]}
    or die qq{unknown door state '$_[0]'};

  return $_[0] = $transition->{$_[0]};

  }