Apache::DBI will not work unless mod_perl was built with:

PERL_CHILD_INIT=1 PERL_STACKED_HANDLERS=1

or:

EVERYTHING=1

during the perl Makefile.PL ... stage.

After installing this module, configuration is simple—just add a single directive to httpd.conf:

PerlModule Apache::DBI

Note that it is important to load this module before any other Apache*DBI module and before the DBI module itself. The best rule is just to load it first of all. You can skip preloading DBI at server startup, since Apache::DBI does that for you, but there is no harm in leaving it in, as long as Apache::DBI is loaded first.