If you wanted a completely automatic method, the following works:

package Node;

BEGIN {
   my $idx = 0;
   require constant;
   constant->import($_ => $idx++) foreach qw(
      IDX_PARENT
      NEXT_IDX
   );
}
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package ElementNode;

BEGIN {
   our @ISA = 'Node';

   my $idx = __PACKAGE->SUPER::NEXT_IDX();
   require constant;
   constant->import($_ => $idx++) foreach qw(
      IDX_NAME
      IDX_ATTS
      NEXT_IDX
   );
}
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Again, 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."