This turns on the GV's UTF8 flag in sv.c when the GV is stringified.
This works the same way overloading and references work, in that the
SvUTF8 flag is only valid immediately after SvPV.
For Nick's much more detailed explanation, see
http://www.nntp.perl.org/group/perl.perl5.porters/2011/07/msg174703.html