While running 'make test' on mod_perl, I got test failures accompanied by more error messages about the missing DBI/connect.al. So I reinstalled DBI from scratch, restarted Apache, and now everything works. I could have sworn there were no errors when I reinstalled DBI prior to recompiling mod_perl, but perhaps I missed something.
What puzzles me is that, now that things are working, I still don't find 'connect.al' anywhere on my computer, though apparently Perl has stopped asking for it. What should I learn from that?