In my (Ubuntu) installation, @INC holds an entry /usr/local/lib/site_perl which, as far as I am aware, is intended to hold such "common modules"

Likewise...I did notice that in my @INC but didn't investigate if it actually exists. It didn't, but it does now 🙂
Problem solved!

I've gone down this route instead of SetEnv because some of the common modules may be used outside of Apache at some point in the future.