now that's extra weird, because introspecting with Devel::Peek reveals that the implementation is (falsely) differentiating between different kinds of 0s and NULLs.

The "other kind of 0 or NULL" is an undefined scalar.

exists differentiates between what Dump shows as SV = 0 (underlying NULL in the array) and everything else (underlying non-NULL in the array).

Say you have this variable:

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

Devel::Peek shows

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
[download]

(Replaced the common start of the addresses with ** to help with readability.)

The underlying array buffer of @a looks like this:

  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
                        +-------------+
                        | ...         | 
                        +-------------+
[download]

(Technically, the body of an SVt_IV scalar is part of the same memory block as its head, but illustrating that would just complicated things needlessly.)

• SV = 0 indicates a NULL pointer in the array.
• SV = followed by anything else indicates there's a non-NULL pointer in the array.

What follows SV = is the type of scalar.

• SV = NULL indicates a scalar of type NULL (SvTYPE(sv) == SVt_NULL). The only value a scalar of this type can hold is undef. my $x; creates such a scalar.
• SV = IV indicates a scalar of type IV (SvTYPE(sv) == SVt_IV). It can hold one of the following: undef, a signed integer, an unsigned integer or a reference.

The second number in SV = TYPE(0xBBB) at 0xHHH is the address of the scalar's head (i.e. of the scalar itself), and the first number is the address of the scalar's body. Scalars of type NULL have no body. As such, the pointer to the scalar's body in such scalars will be NULL (0x0).

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