is it possible to have a failing eval to return a null $@?

I don't think so. If an eval fails, a die message will be set, either because of a fatal error, or an explicite die. In the former case, I don't think it ever happens there's an empty message (except from a bug). In the latter case, if the message ends with a newline, it's not empty, and if it doesn't end with a newline, Perl will add the filename and line number the die happened to the message.

Abigail