I think a slightly more flexible approach would be to use $MIME::Entity::BOUNDARY_DELIMITER and set that to \r\n (if it's not set already). That way, people (still) using Apache Axis can configure their client to be compatible with that weird notion of line delimiters while the rest of the world can use \r\n as god intended.

You can add that patch to rt://MIME::tools, possibly with a link back to this discussion and that other ticket mentioning the wrong kind of newlines being hardcoded. Even if the maintainer ignores that ticket+patch, at least others can find it there and apply it locally.