With MySQL replication one DB serves as the master and others as children. Updates, inserts, and deletes should only be done on the Master (which pushes the changes down to the children), but selects can be done on any of the DBs. This works well because for many tasks because in many instances DB access is much more read-oriented than write-oriented.

You can even do something similar with the DBD::Multiplex. It allows you to have a single database handle (from your program's point of view) that connects to multiple DB's on the back end. It'll use any of the DB's when doing a select, but any updates/inserts/deletes will be performed on all teh DB's.