sub modify {
   my ($str) = @_;

   my ($head, $tail) = ( substr($str, 0, 1), substr($str, 1, length($str)-1) );

   return length($tail) ? map { (lc($head) . $_, uc($head) . $_) } modify($tail) : $head;
}

printf("$_\n") foreach ( modify('abcdef') );