Yes, a good tip. I've found that changing some flags (is_this_type_of_content, is_that_type etc.) from setting their default value explicitly, so instead undef is used as the default value, saves a lot of memory -- as the key and value just don't have to be set at all for this to work.

I think I'll have to serialize the dependencies array into something much smaller, probably using (at the easy end) a serialize-into-string trick and (at the heavier end) some kind of Flyweight-pattern-style trickery.