Plack should act as a middleware between your code and the web server, there shouldn't really be much need for refactoring. In fact, it looks like it already has a handler: https://metacpan.org/pod/Plack::Handler::Apache2. Sorry, this is going the other way, plack -> mod_perl. Regardless, I think the Plack documentation mentions somewhere about routing everything properly through Plack to your existing mod_perl app code: https://metacpan.org/pod/PSGI::FAQ#I-have-a-CGI-or-mod_perl-application-that-I-want-to-run-on-PSGI-Plack.-What-should-I-do

In terms of persistent database connections, I'm not sure how this is tied to mod_perl. However, I recommend looking at DBIx::Connector as it has a lot of really good connection logic built into it while still being "just DBI".