PVIV

• Space for PV and an IV
• Flags say which are valid

    perl -MDevel::Peek -le '$a="question"; $a = 42; Dump($a)'
    SV = PVIV(0x80f7830) at 0x810b4b4
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 42
      PV = 0x80f6090 "question"\0
      CUR = 8
      LEN = 9
  

• Logic: "if it was a string once, it probably will be again"