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

my $f = fib();
$f(10).say;