Thanks, I'll try comp.editors. I wanted includeexpr because it will let me use, say, gf to go to a used or required file directly. In 5.7, there was no way to translate Perl's :: to / or to tack on the .pm.
I came up with this ftplugin file which works pretty well so far.
Updated: moved to snippets, added link. This should make it easier to find and maintain.