$ perl -MDevel::Peek -e " use bytes;  $f=chr(400); utf8::upgrade($f); Dump $f "
SV = PV(0x3e6a0c) at 0x98a474
  REFCNT = 1
  FLAGS = (POK,pPOK,UTF8)
  PV = 0x3ef4e4 "\302\220"\0 [UTF8 "\x{90}"]
  CUR = 2
  LEN = 3