First thing to do is in a terminal type "which -a perl", without the quotes. That will tell you which perl is being found first in a search of your $PATH. If you installed your own perl, it is likely that /usr/local/bin/perl will be the first answer.

If you installed your own perl, you must now start using cpan to install modules. Try this as root: "cpan install DBI" and see what happens. It should install all dependencies needed by DBI first. Watch the terminal and see what transpires as cpan works.... it is quite amazing.