> i tried 

    > sub flip { $@_[0] = !($@_[0] };

##</code><code>##

    sub flip { ${ $_[0] } = !${ $_[0] } }

##</code><code>##

    use Tie::FlipFlop;

    tie my $flipflop => Tie::FlipFlop => qw /Red Green/;

    print  $flipflop;      # Prints 'Red'.
    print  $flipflop;      # Prints 'Green'.
    print  $flipflop;      # Prints 'Red'.