gistfile1.txt

multi fib($x) {
    given $x {
        when * < 0 { die "non-negative arguments required" }
        when %memo{$x} { %memo{$x} }
        default { 
            my $res = fib($x-1) + fib($x-2);
            %memo{$x} = $res;
        }
    }
}

# or better yet:

my @fib := 0, 1, *+* ... *;
sub fib($x) { @fib[$x] }