HERE're my attempts at getting to know and understand:
#!/usr/bin/perl

use strict;
use warnings;

use XML::Twig;
# perl test_thandi CCC '@n=AAA'
# perl test_thandi CCC '@n="AAA" or @set="y"'


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

XML::Twig->new( TwigHandlers =>
   {
#   qq{AC[\@n="$ac_n_value"]/AI[$ai_att_cond]} => \&print_ai_data },
#   qq{AC[\@n="$ac_n_value"]/AI[$ai_att_cond]/ID[\@n=$id_att_cond]} => \&print_id_data },
#     ID => \&id_print , #list all IDs desc <desc> </desc>
#     AI => \&ai_print ,
     qq{AC}    => \&ac_print,  #list all ACs <desc> </desc>
     qq{AC/AI} => \&ai_print, #list all AIs <desc> </desc>
   },
   pretty_print => 'indented',
#   keep_atts_order => 1,
#   do_not_chain_handlers => 1
) ->parsefile( "custs.xml");


sub ac_print
{
   my( $t, $ac)= @_;
   my $Desc = $ac->first_child('Desc')->text;
   print "AC DESC:   $Desc\n";
   print "=======================================\n";
}

sub ai_print
{
   my( $t, $ai)= @_;
   my $Desc = $ai->first_child('Desc')->text;
   print "AI DESC:   $Desc\n";
   print "----------------------------------\n";
}

sub id_print
{
   my( $t, $id)= @_;
   my $Desc = $id->first_child('Desc')->text;
   my $What = $id->first_child('What')->text;
   my $AR   = $id->first_child('AR')->text;

   print "--> DESC:   $Desc\n";
   print "What:   $What\n";
   print "AR:     $AR\n";
}