I'm setting INSTALL_BASE because MacPorts does not contain updated versions of every module I need to install, and their recommendation is to put non-MacPorts-installed Perl modules somewhere besides in the MacPorts /opt/local/lib/perl5 tree. And because I have some locally-developed modules that need a home. So anything that comes straight from CPAN goes into INSTALL_BASE. Isn't that the point of that?

Thanks for the link on per-project perl installs. That's an approach I hadn't thought of, I'll give it some thought. But as I replied above regarding Perlbrew, I'm trying to keep the number of perls at a manageable level, and I think I may have already crossed that threshold!