$ perl -MDevel::Peek -E '$x = pack("B8", "10000000"); Dump($x); utf8::upgrade($x); Dump($x)'
SV = PV(0x923c0dc) at 0x924ebb8
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x9253a80 "\200"\0
  CUR = 1
  LEN = 4
SV = PV(0x923c0dc) at 0x924ebb8
  REFCNT = 1
  FLAGS = (POK,pPOK,UTF8)
  PV = 0x9253a80 "\302\200"\0 [UTF8 "\x{80}"]
  CUR = 2
  LEN = 4