U+5F4C as a regular Perl character string (UTF-8 flag on):
SV = PV(0x629098) at 0x645530
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK,UTF8)
  PV = 0x682d40 "\345\275\214"\0 [UTF8 "\x{5f4c}"]
  CUR = 3
  LEN = 8
=> is alphabetic

same character in Shift-JIS legacy encoding (UTF-8 flag off):
SV = PV(0x629098) at 0x645530
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x75b5d0 "\234\\"\0
  CUR = 2
  LEN = 8
=> isn't alphabetic