DBI is a wrapper for unified access to all sorts of databases, including flat files. The kind of database is determined by the connect string you use for DBI. By writing to DBI's commonest interface, you can switch db types as need arises, with minimal disruption.

As you suggest, flat files don't scale well for speed. They are easily exported to other db types. DBI always costs a little speed, but repays the investment with easier maintainance and modification.