in reply to Sending email with both html and plaintext
Are you looking at what you are sending or the way the email client displays your sent message. Usually if you have both text and html, a html enabled browser will show the html. (Note: I have never used either MIME::entity or Mail::Mailer).
On my email client ( Thunderbird ), you can type the ^U to see what is in the email. If that doesn't work you can save the file and using your favorite editor view the contents.
If the text and html are different then you may have to send a dummy text first, then the html and then the text data.
"Well done is better than well said." - Benjamin Franklin