First, take a look at your local man flock docs: I'm assuming you are on a unix system. Here's some quick notes about file locking:

• Know the difference between shared and exclusive locks, as well as blocking vs. non-blocking.
• Perl does not necessarily know what LOCK_SH and the others are. Define them in a sub or just use the numeric values.
• Remember that *all* your processes should honor (and check for) the locks, or all bets are off. This includes any command prompt programs!
• When in doubt, open the file in read/write mode ("+< $myfile")
• Reading a file: open read, lock, read, close (also unlocks)
• Writing a file: open read/write, lock, read, seek (beginning), truncate, rewrite, close
• Appending a file: open append, lock, seek (end), write, close
• Use sleep() to test your locks: have one process lock it, then sleep for 30 seconds, while you try to open it with another process. The second should be forced to wait for the first.
• Avoid lockf. Flock is your friend. :)

Finally, you say the file is being "constantly read from and appended to." Isn't it also going to be rewritten occasionally as people change their prefs and/or passwords? If you are definitely only appending, then you probably do not need a lock for those who are merely reading the file, as the addition of the appended information is not important on a small time scale.