If your predecessor wrote his own templating system, then it is likely that he also wrote a results caching system, and that system has bugs in it.

I would say you have some detective work ahead of you to find that caching system, and either fix the bugs so that cached results expire more quickly, or rip it out and replace it with a standard library from CPAN.

I would start by greping the filesystem for old cached pages, that should not be valid any more. Once you find them, you can then grep the perl source tree for references to those files.

I don't think the back end database, or the database access library you are using has anything to do with your problem.