See Mail::Message::Body
See SYNOPSIS in Mail::Message::Body
The body (content) of a message can be stored in various ways. In this documentation you will find the description of extra functionality you have when a message is stored as a single scalar.
Storing a whole message in one string is only a smart choice when the content is small or encoded. Even when stored as a scalar, you can still treat the body as if the data is stored in lines or an external file, but this will be slower.
See DESCRIPTION in Mail::Message::Body