-=! and -=!! - flathead +=! and +=!! - phillips *=! and *=!! - torx x=! and x=!! - pozidriv (http://en.wikipedia.org/wiki/Pozidriv) This is a conditional "set to empty string" operator (the string equivalent of the torx): $x x=!! $y is same as $x = '' unless $y; $x x=! $y -- $x = '' if $y;