flock (DATAFILEIN, 8);
    if (open(DATAFILEOUT, ">$catagory.dat") ) {
        flock (DATAFILEOUT, 2);
[download]

Here you unlock the file, then open and clobber it, then lock it again. How do you know some other process didn't add new data to the file in between? Two better solutions would be:

• Open the file read-write, with "+<$category.dat". Do all your reading, seek back to the beginning, truncate the file, and then write the new data back out.
• Use a separate lock file, ">$category.lck". Don't unlock that file until your update is finished. With this solution, you can write the data back to a temporary file, $category.tmp, then rename it to $category.dat. That saves memory, because you don't need to store everything in @temp, but maybe your application doesn't have enough data for that to matter much.

You shouldn't have any trouble getting the viewer to expire old data. You might want to read in all the data and do the necessary expirations first (the code for this part could be reused), and then check to see if there's anything left to display.

Update: It would be a good idea to have the viewer get a read-lock on the file, too.