Not directly related to your mail sending issue but I'm including it because I always find it helpful when people make constructive criticism of my code...

Your script makes use of CGI::Carp through

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

But it doesn't call the warningsToBrowser(); method. This method works differently to fatalsToBrowser. The latter displays the error as part of the HTTP 500 screen. However, for a warning, CGI::Carp can't display a message that early, in other words, before the browser gets the HTTP headers. So you have to choose when to send any warning after the HTTP headers by calling warningsToBrowser(1); (with any true value).

The warning manifests as an HTML comment in the webpage at the point you call the warningsToBrowser(); method. It is usually best to do it at the end so the warning appears at the bottom of the HTML source code. This saves you having to hunt for it!