Perl6 also has optional typing:

  sub fib (Int $n) returns Void {
    my Int ($a, $b) = 0, 1;
    while $a < $n {
      say $a;
      ($a, $b) = ($b, $a + $b);
    }
  }

NB. At moment Void doesn't appear to be implemented in current version of Rakudo. Removing returns Void will make it work!