One module I'd like to point out that can come in really useful for cross-database applications is DBIx::AnyDBD. This follows the principle laid out above - knowing your database is extremely important, and so you should have access to it's features.

But beyond that, it ensures that you can always port your application to a new database in a clean and consistent manner, and in a way that your users won't have to care about (apart from changing the DBI connect string). It does this by simply providing a subclass for each database. So you write your default query stuff in a Default.pm file, and put Oracle specific stuff in Oracle.pm, Sybase specific stuff in Sybase.pm, and PostgreSQL specific stuff in Pg.pm. It then uses simple rules at connect time to give you the right class, and it does this correctly with the Windows abstraction layers ODBC and ADO too.

Another point in its favour is that DBIx::AnyDBD is going to be incorporated into DBI core in the next major DBI revision.