SvOOK 

SV = PV(0x1947c20) at 0x1967ac8
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x1968ed0 "abcdefghijklmnopqrstuvwxyz"\0
  CUR = 26
  LEN = 32

substr($s, 0, 5, '');

SV = PV(0x1947c20) at 0x1967ac8
  REFCNT = 1
  FLAGS = (POK,OOK,pPOK)
  OFFSET = 5
  PV = 0x1968ed5 ( "abcd\5" . ) "fghijklmnopqrstuvwxyz"\0
  CUR = 21
  LEN = 27