I'm not at all sure about this, but I believe Perl's special variables are more like aliases or place-holders to internal variables than normal scalars. That would explain this abnormality.

A good example (the only one I know of, really) is $|. It can assume only the values of 0 or 1, irrespective of what you try to set it to:

  print $|=5, ".\n";
  __END__
  1.
[download]

The reason for this is that the C code that handles assignment to $| does something similar to:

    if (value assigned to $| is true)
      return 1
    else
      return 0;
[download]

This allows the following magic with $|:

    print --$|, "\n" for 1 .. 10;
    __END__
    1
    0
    1
    0
    1
    0
    1
    0
    1
    0
[download]

Again, all of this is just speculation ;)