Sending a TEXT attachment (read text/plain in Mime speak) as the only part of a multipart message has its advantages.

You can expect that long lines (read links) aren't broken apart by the mailer software non-reversible, as it will happen with such beasts in text/plain messages often.

Anyway, the correct Type for such a message is multipart/mixed, see also the corresponding example in the MIME::Lite doc.