Thanks, that example of using code in a regex is exactly what I was wondering.

The perlre page states that (?{ code }) is always successful, but also says that it may be used in a conditional match.

So I'm guessing that if used alone, the code has side-effects only and always succeeds. But if used as the condition of a (?(condition)yes-pattern[|no-pattern]), then it does indeed use the result as the condition.