CGI/html question: How to write file with filename randomly chosen by javascript?

I have the following cgi code I need to modify:

open(OUT, ">files/update.txt")

Code (markup):

The above deletes everything in the file update.txt. I then have more code that fills out update.txt with information from a form.

Now for a contest I'm doing, what I am trying to do is make a new file for each entry in a contest. I need to write a unique file for each file in a contest. The file name will be gotten by javascript,

<script type="text/javascript">
     file_name=new Date();
     document.write('<input type="hidden" name="'+file_name+'">');
</script>

Code (markup):

How do I get my cgi script to write a file with this filename?

Sorry for the nooberific post. I need to get this contest going by tonight.