You can't modify $1 because it's a read only variable. It doesn't make sense to make it a writeable variable because it's only ever supposed to contain the first capture from the most recently successful match.

That may not be the most convincing explanation, but that's the official reason. If you could modify the magic global captures, should or shouldn't it affect the matched string itself? That's pretty tricky and, pragmatically, it's a stone better left unturned.