Apache::Session does some scary things with persistence. In particular, it maintains an exlusive lock on the session data until the hash gets destroyed, which can mean that every other request for this session (including images if you aren't careful) will wait. The hash will not get destroyed while there is any existing reference to it. That's probably what's tripping you up.

If you really want to use Apache::Session rather than something simpler like MLDBM::Sync, I suggest you treat it as a hot potato, i.e. create the hash, get what you want in and out of it as fast as you can, and get rid of it immediately. Don't leave it hanging around while you do other stuff.