Well, it's pretty easy to reproduce, and the effect is the same in ActiveState 5.8.8 und 5.10.

I used ActivePerl 5.10.0 b1005 above. 5.8.8 gives the same result. Both successfully execute the shell, so $? is not -1, so $! is meaningless.

OTOH, this would still make it indistinguishable from a program which deliberately sets that exit code..

Yes.