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."