This looks like a MIME encoded email message to me, so the best thing to do, IMO, would be to use a MIME parsing module.

The next best thing is to extract the boundary from the Content-Type header, in this case "BlatBoundary-Yo7hjfnfSSAUIn61G" — but it'll be different every time; and use this as a pattern to split up the message, something like

/^--\Q$boundary\E$/
[download]

(or /^--\Q$boundary\E$/m if this is to be used with split)

I'm not 100% sure that is how a MIME message is split up.

And if Blat creates mail messages with binary files embedded, that would be enough reason for me not to use it any more. One day, it'll blow up in your face, as I'm sure it's legal for mail programs to treat the contents as plain text — thus: changing of the line ending byte sequence is allowed. You can create, and mail, using one of several methods, MIME messages with MIME::Lite.