In a bit more detail than above:

The signal comes in, interrupting select inside of can_read. Between that op and the next, the signal handler is called. It calls waitpid until it returns an error, setting $! to ECHLD. The signal handler returns, and nothing changes $! between there and when you check it.

By the way, there was talk of having certain variables automatically localised by the signal handler, including $!. I don't know if that's already in 5.12 or 5.14.