You can report this to p5p with the perlbug utility.

My educated guess is that the problem is that $! gets set by the last failed system call. Normal Perl code generates a lot of harmless failures, and so you should not expect to have $! mean anything specific unless something just failed.

The flip side of that is that you should not expect $!'s value to be maintained through any complex code, such as the internals of Carp unless a specific effort is made to do so.