It seems more reasonable to tell the regex engine it has an easier job, close to where it is invoked rather than converting the result.

Note that a regex match has side effects like setting $0, $1 etc., and the full match object is still available in $/ even if the match is performed in boolean context.

So the optimatzion had to a lot smarter than just detecting boolean context (which propagates anyway, so it's not a question of the used syntax).