$ perl -MDevel::Peek -e 'my $x=undef; Dump($x)'
SV = NULL(0x0) at 0x25ea5f8
  REFCNT = 1
  FLAGS = ()
$ perl -MDevel::Peek -e 'my $x=0; Dump($x)'
SV = IV(0x22ae518) at 0x22ae538
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 0
$ perl -MDevel::Peek -e 'my $x=""; Dump($x)'
SV = PV(0xa3e70) at 0xa390e8
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0xa398f0 ""\0
  CUR = 0
  LEN = 10
  COW_REFCNT = 1
$ perl -MDevel::Peek -e 'my $x=defined(); Dump($x)'
SV = PVNV(0x3f230e0) at 0x3fe6108
  REFCNT = 1
  FLAGS = (IOK,NOK,POK,pIOK,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x3f6a860 ""\0
  CUR = 0
  LEN = 10