in reply to I made a small but useful improvement to a module now what?

Think of CPAN as being essentially dumb storage for files, it's not a source code control system. If the modules metacpan page has 'Repository' link on the left hand menu, there should be a link to a working version control system (github/lab etc). Failing that I'd suggest creating a patch (The Ten Minute Guide to diff and patch if you aren't familiar with this) and attaching it to an 'issue' (click the link on the metacpan page, this may take you to or some other issue tracker).