The "num_pages" setting just tells it how many storage buckets there should be, which mostly affects how granular locking will be. I don't think you should be setting this parameter.

Is your cache size big enough? If you go over the size, you will lose data. Maybe you should be using a database (or dbm file) for this instead, since you don't want to lose data.

Incidentally, Parallel::ForkManager is pretty helpful for this kind of thing.