Looking at CLIPS for some Expert System needs I might be having shortly.
The docu so far indicates that it can be used as an embedded system in a larger application, which is what I would need, so that's a good sign.
This is of course all assuming that forward-chaining and not backward-chaining is what you need. (Commercial CLIPS/R2 has backward chaining and C++ as well, but CLIPS 6.x is backward only. ECLIPS had backward chaining, but I don't know if it's available.)
If you don't need speed, just expresivity, why not Perl6::Rule? Maybe because it's not done yet. Conversely, Language::Prolog is over-done, un-maintained.
If you're doing an expert systems project, you should have someone on the project that's done such work before to avoid re-inventing past dead-ends. If you haven't and don't have such a person, I can refer you to friends who were in the field long before my dabbling days.
-- Bill eMail