C:\test>perl -MDevel::Peek -E"$x = '--*--'; substr($x,2,1) = chr(~0); Dump( $x )"
SV = PVMG(0x250e68) at 0x2669c8
  REFCNT = 2
  FLAGS = (SMG,POK,pPOK,UTF8)
  IV = 0
  NV = 0
  PV = 0x252e48 "--\377\200\217\277\277\277\277\277\277\277\277\277\277--"\0 [UTF8 "--\x{ffffffffffffffff}--"]
  CUR = 17
  LEN = 24
  MAGIC = 0x2b66a8
    MG_VIRTUAL = &PL_vtbl_utf8
    MG_TYPE = PERL_MAGIC_utf8(w)
    MG_LEN = -1