My belief is that, on unix, $! mirrors errno exactly. And on unix, errno behaves the way I have described. It is modified (to a non-zero value) by failing system calls and not modified by successful ones. Therefore, on unix, setting $! to zero before making a call to a perl function which (may) make a system call is a valid way of checking for an error.

Can anyone explain why Windows perl behaves as it does, and not like unix, in this respect?

In any case, using readline on Windows seems like a very iffy proposition if you're doing any serious programming which requires tight error checking. I wish that weren't so, because readline certainly is convenient.