There's another reason why you shouldn't put this stuff in more than once place: redundancy. You want to have the code that knows how to do each thing in only one place. This is the "once and only once" rule.

What you can do is make a function that returns a valid database handle and call that from each of your modules. You can have this function do its own caching of the database handle for now (just stuff it into a global) and then when you switch to mod_perl you'll be able to change it to use Apache::DBI.