... wax ... which file or files, precisely, must be discoverable by @INC when trying to use DBD:Pg and having installed the latest O-Reilly DBI and version DBD-Pg-3.3.0 on MacOSX? ... wax ...

If you gotta ask, you should really start over with Installing Perl on OSX - learn.perl.org

In addition you could also go with http://www.citrusperl.com/download.html

For historical reference A Guide to Installing Modules and this link tree ... there is good stuff there , but you don't need to bother with this, let the first link work for you