sub fib(Int $n where $n >= 0) {
    state %cache = 0 => 0, 1 => 1;
    
    if %cache{$n}:exists {
        return %cache{$n};
    }
    
    my $out = fib($n - 1) + fib($n - 2);
    %cache{$n} = $out;
    
    $out;
}

fib(10) == 55 or die "Wrong number";