$ perl -MDevel::Peek -e'Dump(chr(0x7FFFFFFFFFFFFFFF))'
SV = PV(0x5555770a3f00) at 0x5555770d2d50
  REFCNT = 1
  FLAGS = (PADTMP,POK,READONLY,PROTECT,pPOK,UTF8)
  PV = 0x5555770db6f0 "\xFF\x80\x87\xBF\xBF\xBF\xBF\xBF\xBF\xBF\xBF\xBF\xBF"\0 [UTF8 "\x{7fffffffffffffff}"]
  CUR = 13
  LEN = 15