Apache::DBI and DBI::connect_cached work fine, but Class::DBI uses Ima::DBI, which doesn't play nicely with Apache::DBI. This will not really affect you unless you either use transactions (Apache::DBI has a cleanup_handler to do safety rollbacks at the end of each request, but Ima::DBI prevents it from working) or use something like Class::DBI::mysql which will try to load open a database connection when you use it and pull it in from your startup.pl (before forking). I've written explanations with sample code and workarounds on the Class::DBI mailing list and the mod_perl list.