I responded on the mod_perl list too. My suggestion is to abandon trying to make this work from mod_perl. The best thing to do would be to create a daemon process that listens for connections from your mod_perl handler and returns either image data, or drops files someplace where you can send img src links to.

This daemon would also be in charge of closing files left open for more than say 10 minutes (the user has moved on by that point).

None of this is going to scale, by the way. Converting PDFs to images isn't really something you want to do on the fly. Sure, it'll work, for the most part, but unless you're creating the document dynamically and then also creating the thumbnails dynamically from that, it's going to be more efficient to convert all of your PDFs to images ahead of time.