I would highly recommend against flat files if you are doing anything with any kind of volume. A tied DBM hash should be your last resort for if you don't have access to any database usable with DBI. With a flat file, you're going to have big locking issues.

Beyond that and the obvious (like CGI.pm), one module you're definitely going to want to use is Date::Manip. It will save you a lot of time and energy in dealing with the various date formats people might put in.