Mail::Mailer::sendmail is one of the back-ends for Mail::Mailer, and shouldn't be used directory. A good indication for that is the lower-cased 's' in 'sendmail'.
When you look at the Mail::Mailer manual page, you see how to use it. You can also use Mail::Send. The general idea is:
$mailer = new Mail::Mailer;
$mailer = new Mail::Mailer 'sendmail';
print $mailer "This is a very short message";
Mail::Send helps you constructing the headers.