Actually, the answer you seek can be found in the enigmatic cliffhanger ending of Flip Flop II, heralded by audiences and critics alike as being the most poignant, moving, and epic episode of the Flip Flop trilogy released to date:

                 ... sub
                  a   {$
                   _ ^=1}
[download]

Our trustworthy Ovaltine decoder ring translates this cryptic message to reveal the following bit of wisdom:

sub flip_flop {
  $_ ^= 1;
}
[download]

Of course, you shouldn't mess with globals such as $_ in real code, but the principal is what matters here :-)

warning: "Flip Flop" is a trademark of CatFood Entertainment in association Dreamworks SKG and Evil Media Conglomerate Incorporated. All our trademarks are belong to us, not you. Duh.

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print