use DB_File; with the $DB_HASH and allow keys to have multiple values.

Make the the keys the date stamps, and the values user names.

Now you can do partial matches, and can have a many-to-one relationship (on 2003-02-01 , 10 users joined, 4 left)

Since you can have multiple values, you can make the values contain if a user left or joined (pipe it to me), or you can have 2 hashes (%joined, %left), or simply prefix the date strings (left-2003-02-01 or joined-2002-09-22) so you can keep a single %hash.

I'd do it this way, cause I love DB_File.