I've sent multi-megabyte files with Net::SMTP--it does work. There are a couple of places you can hit a problem:

The SMTP protocol does put a limit on line length. Lines generally shouldn't exceed 255 characters. (Don't have the standard handy. It might be as high as 1024, but I wouldn't go that high. If your lines are longer, use an encoding to make them shorter. (Quoted-printable's good for this)

Mail is not a binary protocol. Transmitting characters outside the ASCII printable range is a dangerous and unsafe thing. If you have to go in excess of ASCII 127, or below ASCII 32, encode the data. BASE-64 or uuencoding works reasonably well here, or quoted-printable if its only a few characters.

Lines can't start with a period, though Net::SMTP takes care of this, IIRC. That might be your problem. (Once again, MIME encode. Or double-up the period)