Fixed the problem. In case someone searches the forum and finds this... I had been trying to install DBD::mysql by

[root]# perl -MCPAN -e shell
cpan> install DBD::mysql

If I'm understanding this right there are more than one package to install the same driver, but this one I could clearly see was failing because it couldn't make a connection to the test database. It wanted to use root with no password, so the fix was to change the root mySQL password to nothing, install DBD::mysql, and then add the password again. You can read how to do that at http://www.mysql.com/...ault_privileges.html.

... and it installed fine. mysql now shows in the "modules installed" test:

[root@www /root]# perl test_dbi.pl
Here's a list of DBI drivers:
ADO
ExampleP
Multiplex
Proxy
mysql