Use the more general sort_children, provide a subroutine that returns the TabOrder if it exists, and zero otherwise:

    $element->sort_children(sub {
                                   my ($elt) = @_;
                                   my $to = $elt->first_child('TabOrde
+r');
                                   $to ? $to->text : 0
                               },
                               type => 'numeric');
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