As the solution is buried fairly deeply in this thread, I've provided links here.

++choroba provided the basic information for the solution: "Re^5: require() @INC hooks problem [non-Moose]".

How I implemented the solution: "Re^6: require() @INC hooks problem [non-Moose]".

And again, many thanks to ++choroba. I had spent many hours, on and off since before Christmas, trying to nut this one out.