merlyn has written an article on it, which is usually referenced when these sorts of questions come up:

Watching long processes through CGI

While perhaps not as robust as merlyn's method, there's a much simpler way to do it. Just turn on autoflush ($| = 1) and have your script start outputting to the browser right away. Then update your progress bar every so often while your script runs. Here's a simple example:

#!perl -w
use strict;
use CGI;

$| = 1; # turn on autoflush

print CGI->header;
print "Progress: ";

for (1..10) {
  print '.';
  sleep 2;
}

print "\n<br><br>Done!";
[download]

This method does not always work. For instance, I have been working on a Web App which did something similar. We recently changed the system architecture so that the CGI now sits behind an Apache Reverse Proxy. I cannot get the Reverse Proxy to unbuffer, so this method no longer works. I wonder if a regular Forward Proxy might not do the same thing - probably depends on the proxy server. Just thought I'd mention it.

--
edan

I print out

<img src="rotating_thing.gif" name="progress">
[download]

<script>
  document.images.progress.src='finished.gif';
</script>
[download]

It at least gives the user something to look at while the process is going on.

Obvious question -- why does your process take so long? Why don't you ask us if we know a way to speed it up?