I found my module installation problem: it has nothing to do with CPAN.pm (thank god).
It comes down to how different people do things. I do a lot of Perl and install a lot of modules on my Powerbook, so I chown-ed the Perl modules directories to my normal, non-priveleged user. I can install modules for all users and do so without any special priveleges. (In past lives I have also used a "perl" group to do that when several people should have that ability).
Not everyone does it that way. Maybe nobody else does, even. If I don't own the directories as my regular user, I invoke super-user privs:
sudo cpan Local::Foo
Well, in my present hair-pulling problem, that's a whole different ballgame since the module writes and reads files, and they need a particular set of priveleges (not up to me). The super-user gets to do a lot of things the priveleges don't imply.
albook_brian$ ls -l
---------- 1 brian brian 0 5 Sep 14:54 not_readable
-r-------- 1 brian brian 0 5 Sep 14:54 readable
albook_brian$ perl -le 'print "Readble!" if -r shift' not_readable
albook_brian$ sudo perl -le 'print "Readble!" if -r shift' not_readable
I'm not sure how this translates to other operating systems, and I had previously thought the usual "stoopid Windows can't get it right" mantra, but even my Mac OS X and FreeBSD boxen do the same thing. I'm guessing that some OSen must do it differently or I would have received more failure reports.
Or maybe everyone is installing things in private directories. Or the module is crap and no one uses it.