cynix has asked for the wisdom of the Perl Monks concerning the following question:

  I want to store some usernames and passwords in a file on my site. The username and password combined is only 20 chars. Now, if I have 4000 users stored in a flat file (each user-password pair getting one line), the file size is 80Kb or so. But, if I use a DB_File (with user-key, password-value pairs), the file size is around 180Kb or so.

My question is:

If data corruption is a non-issue (only admin can change data), and speed is the only issue; which storage format should I use? Especially since I'm only looking for one username.