>perl -MDevel::Peek -we "Dump 1; sub foo{Dump $_[0];$_[0]=1} foo(1)"
SV = IV(0x653478) at 0x653488
  REFCNT = 1
  FLAGS = (IOK,READONLY,PROTECT,pIOK)
  IV = 1
SV = IV(0x653358) at 0x653368
  REFCNT = 1
  FLAGS = (IOK,READONLY,PROTECT,pIOK)
  IV = 1
Modification of a read-only value attempted at -e line 1.

>perl -MDevel::Peek -we "Dump 1+1; sub foo{Dump $_[0];$_[0]=1} foo(1+1)"
SV = IV(0x2612cf8) at 0x2612d08
  REFCNT = 1
  FLAGS = (PADTMP,IOK,READONLY,PROTECT,pIOK)
  IV = 2
SV = IV(0xfcb140) at 0xfcb150
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 2