Hmm. You're writing to the filehandle MAIL, but you've never opened it. And by your use of Mail::Mailer, you don't want to. If you RTFM, you can basically just lift the SYNOPSIS straight into your script:

$mailer = Mail::Mailer->new;
$mailer->open(\%headers);
print $mailer $body;
$mailer->close;
[download]

Adjust as needed.

Makeshifts last the longest.