First, i recommend to uninstall active perl. ActivePerl is not needed at all because Debian has its own perl packages.
If you followed the howto, perl is already installed and configured on your server. Install the additional perl module with the apt-get command.
To install DBI and DBD-Mysql:
apt-get install libdbi-perl libdbd-mysql-perl
apt-get -u upgrade
to update your server.