There are a few problems with your code:

• first, as mentionned by runrig, you delete $balance and then you flush it. So $balance is deleted, and becomes undef-ed, so you can't call a method on it (are you sure this code worked in 5.6.1?)
• you do a couple of times things like this:

      my $balance30elt = new XML::Twig::Elt('BAL_30', $balance30);
      $balance30elt->set_text($balance30);
      $balance30elt->set_att(Period => 1);
      $balance30elt->paste('first_child', $balance);
  [download]

  The set_text is completely useless (you set the text in the new above, and BTW new XML::Twig::Elt is a deprecated way of calling a method, XML::Twig::Elt->new is the proper way.
  In any case, you can replace all those lines by the more compact

      $balance->insert_new_elt( first_child,     # position
                                'BAL_30',        # tag
                                { Period => 1 }, # attributes
                                $balance,        # content
                              );
  [download]

  And actually it looks like you don't even need to delete then re-create the BAL_30 (and BAL_60) element. You could just do:

     $balance->first_child( 'BAL_30') # get the element
             ->del_atts               # if it had atts originally
             ->set_att( Period => 1)
             ;
  [download]

Does it make sense?

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