Here is what it would look like:

#!/usr/bin/perl

use strict;
use warnings;

use XML::Twig;


my( $ac_n_value, $ai_att_cond)= @ARGV;

XML::Twig->new( twig_handlers => { qq{AC[\@n="$ac_n_value"]//AI[$ai_at
+t_cond]} => \&print_ai_data })
         ->parsefile( "test_thandi.xml");

sub print_ai_data
  { my( $t, $ai)= @_;
    print "DESC: ", $ai->first_child( 'Desc')->sprint, "\n",
          "ID  : ", $ai->first_child( 'ID')  ->sprint, "\n"
          ;
  }
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You call this with the value you want for the n attribute of the AC element, and the condition for the AI element:

perl test_thandi CCC '@n=AAA'
perl test_thandi CCC '@n="AAA" or @set="y"'
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A couple of comments on the code: Perl and XPath strings don't really mix very well: you can use alternate quotes (qq{}) to avoid the collision of perl interpolating quotes and of XPath attribute quotes (or you can use ' instead of " in the XPath expression), but you need to backslash the @ used for attribute conditions in XPath, so it is not interpolated as an array by Perl. An alternate method is to use sprintf to build the XPath expression.

This code loads the entire document in memory, which you may or may not want. There are techniques to avoid this described in the XML::Twig Tutorial.

Also, you need the development version of XML::Twig to be able to run this, you can get it from xmltwig.com.

Below are the queries that I need to formulate based on the xml at the bottom. I'll also have to add a new node e.g starting from the <ID>..</ID> tks before hand thandi

1. List All ACs in xml document
   result should be:
        AC desc AC=CCC, CL=CCC
        AC desc AC=AAA, CL=CCC # these are the <Desc> element just bel
+ow start tag <AC> also need to be able to update the text if requeste
+d.

2. List all AIs in a given AC, eg AC=CCC
   result should be:
        AI desc AI=BBB, AC=CCC
        AI desc AI=XXX, AC=CCC

3. List all <ID> nodes in a given <AI n="XXX">
   result should be:
     a.  ID desc ID=XXX, AI=BBB, AC=CCC
         What XXX ID
         ID_XXX

     b.  ID desc ID=ZZZ, AI=BBB, AC=CCC
         What ZZZ ID
         ID_ZZZ

The based on the user selection, I need to update the requested text w
+ithin an element or add a whole new node. 


<World n="earth" >
  <Space n="XXX">
    <CL n="XXX">
      <Desc>Class description</Desc>
      <Other/>
      <AC n="AAA" set="n">
        <Desc>AC desc AC=AAA, CL=CCC</Desc>
      
        <AI n="AAA" set="n">
          <Desc>AI descr AI=AAA, AC=AAA</Desc>
          <ID n="AAA" set="y">
            <Desc>ID desc ID=AAA, AI=AAA, AC=AAA</Desc>
            <What>What ID=AAA AI=AAA, AC=AAA</What>
            <AR>ID_aaa</AR>
          </ID>
            
          <ID n="BBB" set="y">
            <Desc>ID desc ID=BBB, AI=AAA, AC=AAA</Desc>
            <What>What ID=BBB AI=AAA, AC=AAA</What>
            <AR>ID_bbb</AR>
          </ID>
        </AI>
      
        <AI n="BBB" set="y">
          <Desc>AI desc, AI=BBB, AC=AAA</Desc>
          <ID n="AAA" set="y">
            <Desc>ID desc ID=AAA, AI=BBB, AC=AAA</Desc>
            <What>What AAA ID=BBB, AI=BBB, AC=AAA </What>
            <AR>ID_bbb</AR>
          </ID>
        </AI>
      </AC>

      <AC n="CCC" set="y">
        <Desc>AC desc ACC=CCC, CL=CCC</Desc>
        <AI n="AAA" set="n">
          <Desc>AI desc AI=BBB, AC=CCC</Desc>
          <ID n="AAA" set="y">
            <Desc>ID desc ID=AAA, AI=AAA, AC=CCCC</Desc>
            <What>What AAA ID=AAA, AI=AAA, AC=CCC </What>
            <AR>ID_aaa</AR>
          </ID>
        </AI>
      
        <AI n="XXX" set="n">
          <Desc>AI desc AI=XXX, AC=CCC</Desc>
          <ID n="XXX" set="y">
            <Desc>ID desc ID=XXX, AI=BBB, AC=CCC</Desc>
            <What>What XXX ID </What>
            <AR>ID_XXX</AR>
          </ID>

          <ID n="ZZZ" set="y">
            <Desc>ID desc ID=ZZZ, AI=BBB, AC=CCC</Desc>
            <What>What ZZZ ID </What>
            <AR>ID_ZZZ</AR>
          </ID>

        </AI>
      </AC>

    </CL>
  </Space>
</World>
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Are you the same person that asked the original question? You don't have the same user name.

What did you try? Did you use my previous answer as a base and try changing it to do what you want? Is there anything I should add to the tutorial or the docs of the module to make them clearer? Thanks.