I think book vs. book(learning_perl) is fundimental. A fact is not a simple symbol, but a nested structure that is to be matched, possibly with a pattern. Furthermore, this is the identical structure as a rule such as gives.

Have you looked at CLIPS? It's been decades since I read the manual, but forgetting the forward-vs-backward chaining issues, I'm thinking that the way it represents knowledge and gets this into a C program would be worth looking at. It's easier to build up ad-hoc data structures in Perl...

I think that knowledge representation is key. It doesn't matter whether it's persisted and how, but simply decide how everything is hooked up, to represent arbitrary rules and attributed facts and variables.