I have a bunch of little modules in the My:: namespace. This avoids namespace collisions, makes it really obvious when I am using personal code (especially because I now explicitly import all subs) and makes it easy to find and move non-CPAN modules in the filesystem, for example when migrating to a new machine.

Examples: My::Uid for unique IDs (usually in a Webapp context); My::Logfile qw(/path/to/logfile) for easy debug logging; My::Security for easy password encoding (with a default random personal salt).

I'm confident there are probably CPAN solutions to these problems or, when there are not, I'm only saving myself a few keystrokes. I figure I can always map my own code (when it exists) into a wrapper around the Proper Module. And I certainly would never ship any code off to the world that included any of My:: modules -- I would simply find a substitute off CPAN or copy the code over.