Do you turn off use strict; and use warnings; when they start issuing messages?
Actually, I was hoping for a "Wide character in print ..." warning (and a correct Content-Length header). That would have solved the whole mystery ...
But enough of that. The funny thing is, I'm currently writing a web framework and one of the design decisions I have not made yet is whether to allow a character string as the response body or not, because of this whole bytes / print issue. So I would really like to see a real world example where Perl gets the length in bytes of a string wrong when compared to the number of bytes actually printed, assuming no PerlIO layer and no direct manipulation of the string's UTF-8 flag.