sub fibonacci {
    my $fib = shift;
    return $fib if ! $fib || $fib == 1;
    my ($low, $high) = (1, 0);
    my $iterate = sub {
        ($low, $high) = ($high, $low + $high);
        return $high;
    };
    $iterate->() for 1 .. $fib;
    return $high;
}

print fibonacci(0), $/;