Mail::Mailer does not do any of the MIME work for you. It just sends the text you give it as the body of the mail, as is. If you want to do MIME encoding (including setting Content-Type's) you need to build the MIME message yourself.

Check out MIME::Lite. Update: MIME::Lite will not only do the encoding for you, it will also send the mail using either sendmail, or Net::SMTP (just like Mail::Mailer). So you should only need MIME::Lite. :-)

bbfu
Seasons don't fear The Reaper.
Nor do the wind, the sun, and the rain.
We can be like they are.