I suspect your problem may be that you are not sending a blank line between the header and the body, so your whole body is being sucked into the header and confusing either the SMTP server or the user agent. I think this is why it decided it was MIME-formatted as well, even though you have not generated a Content-type.

Try changing your second-to-last line to:

$smtp->datasend("Subject: Status Report $subject_date\n\n");
[download]

There may well be a limit to how much data you can send with no newlines embedded, but I don't think the actual length of the string itself matters as long as it's divided up by newlines every so often (about 6 lines of text for some user agents I've used). I've successfully used Net::SMTP to send a packet of 20 encoded pngs all sucked into one string, though so I know the string can be long.