I think something about that line is not strict enough for the regular expressions to match, but I don't immediately see what makes Email::MIME::ContentType dislike that line.

Update: It seems that the handling of Email::MIME::ContentType does not like ? in the string (and the test suite does not contain anything like name="?UTF-8?foobar"). RFC 2048 allows such values (I think), so I think that this causes the bug (but I haven't written a proper test case to reproduce this). I haven't reported the issue yet, so if anybody wants to beat me to the punch (or finds the existing issue for that), feel free to open an issue yourself!