Mail::Sendmail is a module. You don't just use it and hope a shell call to sendmail will work. It has its own way of doing things ie an interface. You need to start looking in the error logs:

$ tail /var/log/httpd/error_log
[download]

$ is shell prompt, don't type it in.

You will see an errol like Can't find Mail::Sendmail in .... along with the prem end script headers error.

See CGI Help Guide