Mysql.pm and DBD::mysql implement two different approaches to communicate with an mysql server. DBD::mysql is built upon the DBI, the generic Perl Database Interface. It brings you an identical interface to a broad variety of databases and is in this regard comparable to ODBC. The advantage of the DBI approach is portability and interoperability. Mysql.pm is the elder species. It was written before DBI was available but inspired by an early draft of the DBI specification. As has been been circulating longer it are more mature and pretty stable. It is also more complete than DBD::mysql.