This is just off the top of my head. It should get you going in the right direction:

You need to see the error message. It's probably in the web server's error log. Ask the web host for instructions, or read their help pages.

If you can't view it, you need to force it onto the web page (instead of the generic error you get).

Put a

use CGI::Carp qw(fatalsToBrowser);
[download]

near the top of the script (before the CGI::Session stuff).

If you still get the generic error, there might be a syntax error in your script somewhere else. Run it on a command line.

/J