Update: I changed around the architecture of my distribution, so that each module was in its "proper" place -- i.e., instead of PolMaker-Rate-AutoLiability/AutoLiability.pm, I'm using the directory structure that will be in place in the installation, PolMaker/Rate/AutoLiability.pm. (I've used the former structure before, without problem, but I noticed in my troubleshooting that no one else around here seems to use it. Hm.)

Still working just fine on OS X; still broken on Linux. Then it occurred to me that since I'm root, there's nothing stopping me from trying to install the various modules in site_perl, right? So, I took out this line from the top-level Makefile.PL:

INSTALLSITELIB => $paths->{cgilib},
[download]

And boom! Lines upon lines of beautiful things like

Installing /usr/local/lib/perl5/site_perl/5.8.0/PolMaker/Rate/AutoLiab
+ility.pm
[download]

So the problem is fixed, in a way. (I'd rather keep these modules local, but I'm happy that it's working, at this point.) But I still don't know why I was having the problem in the first place. The only difference I can find is that the version of ExtUtils::MakeMaker is different: 6.03 on the Mac, and 6.17 on the Linux box.

Any ideas/comments (including "use Module::Build instead, you dolt!"), I'd love to hear 'em.