I presume you don't have access to the server logs, which is a shame. One hosting company I know sends daily extracts from the apache error log by email to their clients.

Anyway, presuming you don't have access to the server logs, put the following line into the CGI script you are debugging:

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

And remember to take it out when you have finished debugging, if you care about security that is. For more on this aspect, see Does fatalsToBrowser give too much information to a cracker?.