Interesting, I had never thought about using ignore_elts to effectively delete elements. That's smart though. You just have to be sure that you don't ever have any other element in datarec. I would maybe add an extra $drec->cut_children at the beginning of add_filename, just in case. Fixed formats seem to have a natural tendency to evolve over time. ;--(

You could get a slightly shorter, and I think nicer looking code, by using the insert_new_elt method:

    $elt = new XML::Twig::Elt('fileoffset','0');
    $elt->paste('last_child',$drec);
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becomes: