If you define fopen in the same package as you use it it works fine. When passing the bareword to the mmm package, however, it gets interpreted as *mmm::F instead of *main::F, so the file has been opened but in the wrong glob.

Given the prototype, this is arguably a bug - the bareword 'F' should be coerced to a glob in the caller's context. However there is no guarantee it is easily fixable - I suspect that it is currently passed as a simple string, and not turned into a glob reference until the actual assignment in mmm::fopen().

Hugo