I don't see the need of calculating all the indexes automatically. I settled on the following: 
package Node;

use constant FIRST_IDX  => 0;
use constant IDX_PARENT => FIRST_IDX + 0;
use constant NEXT_IDX   => FIRST_IDX + 1;

[download]
package ElementNode;

BEGIN { our @ISA = 'Node'; }

use constant FIRST_IDX => __PACKAGE__->SUPER::NEXT_IDX();
use constant IDX_NAME  => FIRST_IDX + 0;
use constant IDX_ATTS  => FIRST_IDX + 1;
use constant NEXT_IDX  => FIRST_IDX + 2;

[download]

If ElementNode needed to access IDX_PARENT, then I'd either add an accessor or I'd add 

use constant IDX_PARENT => __PACKAGE__->SUPER::IDX_PARENT();

[download]

If a field needs to be added, only the indexes in the class where the field needs to be added are changed. 

package Node;

use constant FIRST_IDX  => 0;
use constant IDX_PARENT => FIRST_IDX + 0;
use constant IDX_ISROOT => FIRST_IDX + 1;
use constant NEXT_IDX   => FIRST_IDX + 2;

[download]

This doesn't support multiple inheritance, but it does support traits and (java-like) interfaces. As said by Steve Cook, "Multiple inheritance is good, but there is no good way to do it."

In reply to Re: Problems I've had with array based objects by ikegami
in thread Problems I've had with array based objects by jimt

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.