#!/usr/bin/perl
use 5.026;
use strict;
use warnings;
use XML::Twig;

{
  open (my $OFILE, '>:utf8', 'file.out.xml') or die "$!\n$^E";
  
  my $t = XML::Twig->new(
  twig_handlers => {
    '/keys/key' => sub { makeDuplicates(@_, $OFILE); 1; },
  },
    output_encoding => 'utf-8',
    pretty_print => 'indented',
  );
  
  $t->safe_parse(\*DATA);
  if ( $@ ) {
    die "Error occured in XML data\n$@";
  }
  
  close $OFILE;
}

sub makeDuplicates {
  my ($_twig, $key_element, $OFILE) = @_;
  
  my $text       = $key_element->text();
  my $attributes = $key_element->atts();
  $attributes->{'#CDATA'} = 1;
  
  # '#CDATA' attribute is defined here
  my $lc_element = XML::Twig::Elt->new('lc_key', $attributes, lc $text);
  
  # '#CDATA' attribute is undefined here
  my $uc_element = XML::Twig::Elt->new('uc_key', $attributes, uc $text);
  
  $lc_element->paste(after => $key_element);
  $uc_element->paste(after => $key_element);
  
  $_twig->flush($OFILE);
  
  return;
}
__DATA__
<?xml version="1.0" encoding="utf-8"?>
<keys>
  <key order='01'><![CDATA[One hen]]></key>
  <key order='02'><![CDATA[Two ducks]]></key>
  <key order='03'><![CDATA[Three squawking geese]]></key>
  <key order='04'><![CDATA[Four limerick oysters]]></key>
  <key order='05'><![CDATA[Five corpulent porpoises]]></key>
  <key order='06'><![CDATA[Six pair of Don Alverzo's tweezers]]></key>
  <key order='07'><![CDATA[Seven thousand Macedonians in full battle array]]></key>
  <key order='08'><![CDATA[Eight brass monkeys from the ancient sacred crypts of Egypt]]></key>
  <key order='09'><![CDATA[Nine apathetic, sympathetic, diabetic old men on roller skates, with a marked propensity towards procrastination and sloth]]></key>
  <key order='10'><![CDATA[Ten lyrical, spherical, diabolical denizens of the deep who stalk about the corners of the cove all at the same time.]]></key>
</keys>