Save yourself alot of hassles by checking out the DBD::CSV module or the docs for the DBM databases which come with Perl.

DBD::CSV will let you use SQL queries and inserts against a flat-file, which should save you a nice chunk of work.

Think 'join/split' if you want to try the DBM path, since it kind of lets you read and write a hash from/to disk. There are several DBM modules available, and the choice depends upon what is on your system. AnyDBM_File is a good place to start.

If this is something that is meant to used for something other than experimentation, then after you implement one of the above then start thinking about using a relational database and your server's built-in authentication.