When can I take given as read?

[Piers Cawley]

Suppose I have a simple, single argument recursive function:

  sub factorial (Int $n) {
    return 1 if $n == 0;
    return $n * factorial $n;
  }

Can I write that as:

  sub factorial (Int $n:) {
    return 1 when 0;
    return $n * factorial $n;
  }

NB. Yes, I know it's a pathological example.