Your first stop should always be Apache's error_log. Alternatively you could use fatalsToBrowswer while debugging the problem:

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

This will send the error messages to your browser so you don't have to hunt for them in the error_log. You will only want to use fatalsToBrowser when debugging because it's a security risk to leave it in production code.

-- vek --