A series of comments:

1. I would not assume that the data file had a name ending in .dat. That is the kind of assumption that can change, and then people will be unhappy because their data file got wiped out.
2. You need to set the LOCK_NB flag as well as LOCK_EX or else your first try will hang until you get the lock.
3. In a CGI environment, waiting 1 second per retry is probably a bit much...
4. While you have control of the file, you don't need to be cautious about truncates and seeks. Just open for read, read, close, open for write, and write as usual.
5. Explicitly closing the data file is a good idea.
6. However I like calling local on *FH if I will use FH as a filehandle. Just a good practice...

UPDATE
Others have already said everything that I would have to say other than the fact that my fifth point is that you actually did. I offered you code earlier where the lock went away just by going out of scope. If you use that then you still want to be explicit about closing the date file. I was just saying that it was subtle but important point.