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    > i tried 

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

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

    use Tie::FlipFlop;

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