I can sympathise with you not wanting to aim too high just yet :)

The typical solution to talking to different backends is usually by adding a driver layer (similar to the DBD::* modules for DBI). I've been browsing through the source for Authen::Simple, and the basic design looks very good. It uses Adaptor classes which implement the backend-store specific code behind the public, common API. It may be too abstract for your needs, but I think I can recommend it. It's interesting and educational at least :)