for (my $i = 0; $i < length($foo); $i++) {
   my $bar = substr($foo, $i, 1);
   if (ord($bar) == 15) {
      $bar = chr(10);
   } elsif (ord($bar) == 16) {
      $bar = chr(13);
   } elsif (ord($bar) == 17) {
      $bar = chr(32);
   } elsif (ord($bar) == 18) {
      $bar = chr(9);
   } else {
      $bar = chr(ord($bar)-2);
   }
   substr($foo, $i, 1) = $bar;
}

print $foo."\n";