I find it's good to understand what you're working with. (this being said as I deal with data at work that I have absolutely no idea what it actually means)

Basically, email is sent using SMTP as what it calls a mail object, which is composed of headers, an empty line, and a message body.

Bodys are required to be ASCII, which limits you to 7bits, but someone thought it would be a good idea to send non-text files, so came up with MIME. Using MIME, the body may be identified as being one or more encapsulted objects. To mark the body as being MIME encoded, there are additional headers inserted into the heading of the email message.

There's a fair bit of background information in the MIME::Tools documentation.