You haven't really given me much to go on, so I can't tell you how to fix it, but I can tell you how you might start going about debugging it... Try invoking your cgi script from the command line, manually feeding it values, and looking at all the stuff that comes back, HTTP headers included. To do this, you'll want to instantiate a CGI object with a file handle to a file that contains field value pairs, or with STDIN and type in the values when you run the script.

use CGI;
open(FILE, "<data.txt") or die "gah!";
my $query = CGI->new(FILE);
[download]

Where data.txt contains stuff like...

foo=bar
baz=biz
[download]

This cuts your web server and the browser out of the loop, isolating your debugging just to your script. Make sure that your HTTP headers are coming out looking alright, specifically checking to see that your headers and HTML body are separated by a blank line.