One con of is that each child process will have to maintain its own cache -- written memory will be copied, not shared, between processes.

I'm not interested in using a module to do it because then I don't learn anything.

You are allowed to read the source code of the Memoize module, as well as MJD's article on the subject. Seriously, you're more likely to learn a hundred wrong ways to do something than you are to stumble across a good approach by accident.