For information on javascript check here.

I had the upload handler write the byte count to a session file, and the monitor process which was opened through the javascript on the "Upload" button read from the same session.

CGI.pm also has an upload_hook subroutine which you can use to do this - you don't explicitly need a::r. You might also want to take a look at Apache::UploadMeter , I was using mod_perl2 so I had to roll my own.