my @a;
$a[1] = undef;
$a[2] = 123;

##</code><code>##

SV = PVAV(0x**6adf0) at 0x**99148
  REFCNT = 1
  FLAGS = ()
  ARRAY = 0x**8b9c0
  FILL = 2
  MAX = 3
  FLAGS = (REAL)
  Elt No. 0
  SV = 0
  Elt No. 1
  SV = NULL(0x0) at 0x**69538
    REFCNT = 1
    FLAGS = ()
  Elt No. 2
  SV = IV(0x**696f0) at 0x**69700
    REFCNT = 1
    FLAGS = (IOK,pIOK)
    IV = 123

##</code><code>##

  ARRAY
  @ 0x**8b9c0
  +-------------+       Head
0 | NULL        |       @ 0x**69538
  +-------------+       +-------------+
1 | 0x**69538 --------->| NULL        | Body
  +-------------+       +-------------+
2 | 0x**69700 ------+   | 1           | REFCNT
  +-------------+   |   +-------------+
3 |             |   |   | ...         | FLAGS
  +-------------+   |   +-------------+
                    |   | SVt_NULL    | TYPE
                    |   +-------------+
                    |   |             | 
                    |   +-------------+
                    |
                    |   Head                  Body
                    |   @ 0x**69700           @ 0x**696f0
                    |   +-------------+       +-----------+
                    +-->| 0x**696f0 --------->| ...
                        +-------------+
                        | 1           | REFCNT
                        +-------------+
                        | SVf_IOK|... | FLAGS
                        +-------------+
                        | SVt_IV      | TYPE
                        +-------------+
                        | ...         | 
                        +-------------+