Highly Recursive Functions

        sub fib {
          my ($n) = @_;
          return $n if $n ==0 || $n == 1;
          return fib($n-2) + fib($n-1);
        }

This function is very very slow!
fib(20) computes fib(18) and fib(19)
fib(19) computes fib(18) again, also fib(17)
fib(18) (done twice) also computes fib(17)...
What a waste of time!