Raise warning on multi-byte char in single-byte locale

Raise warning on multi-byte char in single-byte locale

See http://nntp.perl.org/group/perl.perl5.porters/211909

Something is quite likely wrong with the logic if say in a Greek locale,
Unicode characters (especially Greek ones) are encountered.  The same
character will be represented by two different code points.  This
warning alerts the user to this undesirable state of affairs.