$ perl -MDevel::Peek -e'
    $s = "abc"; Dump $s;
    $s = undef; Dump $s;
    undef $s;   Dump $s;
'
SV = PV(0x814fb00) at 0x814f69c
  REFCNT = 1
  FLAGS = (POK,pPOK)         <-- Var contains a string
  PV = 0x81651b8 "abc"\0
  CUR = 3
  LEN = 4
SV = PV(0x814fb00) at 0x814f69c
  REFCNT = 1
  FLAGS = ()                 <-- Var is undefined
  PV = 0x81651b8 "abc"\0     <-- String buffer still
  CUR = 3                         allocated after clearing
  LEN = 4
SV = PV(0x814fb00) at 0x814f69c
  REFCNT = 1
  FLAGS = ()                 <-- Var is undefined
  PV = 0                     <-- undef freed the buffer