First, is this a javascript or a perl question?, i think you might be confused.
But to help you out of your dream : javascript can NOT write to a local harddisk (only a cookie) and that is good!.
Only serverside javascript can write on the (server) disk

Next, your question is not clear to me. what do you want to do? write something to the server? let perl generate some javascript according to the request it got, so it generates text files? or do you want the javascript to generate other params that should be send to te server where it generates different text files?

anyway, perhaps read how (not) to ask a question first.