use strict;
use warnings;
use Devel::Peek;

my $y;
Dump($y);

for my $x (undef, 10) {
    print "\$x = $x\n";
    Dump($x);
    Dump($_) for ( ($x, $y, undef) = 1..10), $x, $y;
}

##</code><code>##

SV = NULL(0x0) at 0x8ad23d0
  REFCNT = 1
  FLAGS = (PADMY)
Use of uninitialized value $x in concatenation (.) or string at ./test.pl line 10.
$x =
SV = NULL(0x0) at 0x8ab54fc
  REFCNT = 2147483499
  FLAGS = (READONLY)
SV = IV(0x8ab987c) at 0x8ab9880
  REFCNT = 2
  FLAGS = (IOK,pIOK)
  IV = 1
SV = IV(0x8ad23cc) at 0x8ad23d0
  REFCNT = 2
  FLAGS = (PADMY,IOK,pIOK)
  IV = 2
SV = IV(0x8adf414) at 0x8adf418
  REFCNT = 2
  FLAGS = (IOK,pIOK)
  IV = 3
SV = NULL(0x0) at 0x8ab54fc
  REFCNT = 2147483500
  FLAGS = (READONLY)
SV = IV(0x8ad23cc) at 0x8ad23d0
  REFCNT = 2
  FLAGS = (PADMY,IOK,pIOK)
  IV = 2
$x = 10
SV = PVIV(0x8ab86ac) at 0x8ad2410
  REFCNT = 2
  FLAGS = (PADTMP,IOK,POK,READONLY,pIOK,pPOK)
  IV = 10
  PV = 0x8af56a0 "10"\0
  CUR = 2
  LEN = 4
Modification of a read-only value attempted at ./test.pl line 12.