The easiest thing to do is look in the httpd error log to see what errors your script is generating.

If that doesn't work, or you need more information, upload a small shell script like this:

#!/bin/sh -x

printf "Content-type: text/plain\n\n"
exec 2>&1

./your_script.cgi
echo "Exit status: $?"
[download]

, make it executable, and run it from your browser. All the errors that would normally go to your screen will show up in your browser. It's a great help for tracking these things down.

Another technique is to upload a small script, say:

#!/usr/bin/perl -w

use strict;

print "Content-type: text/html\n\nThis is a CGI script.\n";
[download]

then keep adding on to it until it stops working. That should give you an idea of what's not working.