The difference is more visible this way:

use Devel::Peek;
Dump("");
Dump(!1);
[download]

The output of which is

SV = PV(0x816d944) at 0x8163c48
  REFCNT = 1
  FLAGS = (PADBUSY,PADTMP,POK,READONLY,pPOK)
  PV = 0x8167830 ""\0
  CUR = 0
  LEN = 1
SV = PVNV(0x814d6a0) at 0x814c600
  REFCNT = 2147483647
  FLAGS = (PADBUSY,PADTMP,NOK,POK,READONLY,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x814d678 ""\0
  CUR = 0
  LEN = 1