You have a couple of options. You could make your code smart enough to call multiple Class::DBI classes and add up the results in perl. Or, you could make your database hide this complexity. Using views would do it, or some kind of partitioning like MySQL 5.1 supports.

You could also do your own SQL generation, but Class::DBI will be useless to you then. In general, Class::DBI is not good for reporting queries. It's really more useful for manipulating individual database rows.