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

##</code><code>##

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

##</code><code>##

  perl -w -e 'use strict; sub flip { $$_[0] = !($$_[0]) }; my $z=1; flip(\$z); print "$z\n"'
  1