#!/bin/perl -w
use strict;
use XML::Twig;

my @results;                                                # does not have to be global, it's just easier

# create the twig see the docs for why to use TwigRoots
my $t= new XML::Twig( TwigRoots => { elt => \&check_elt }); # call check_elt every time an element elt is parsed
$t->parse( \*DATA);                                         # parse the XML (use parsefile to parse... a file)

print join "\n", @results;
print "\n";

sub check_elt                                               
  { my( $t, $elt)= @_;)                                     # $t is the XML::Twig object
                                                            # $elt is an XML::Twig::Elt object
    if( my $child= $elt->first_child( 'child'))             # that's how you navigate the element
      { push @results, $child->text; }                      # text includes sub elements of child
    else
      { push @results, "missing child for elt " . $elt->att( 'id'); }
    $t->purge;                                              # call only if your document is huge
  }                                                         # to free the memory


__DATA__
<doc>
  <dummy>I am a </dummy>
  <elt id="elt1"><child_1/><child>child 1</child></elt>
  <elt id="elt2"><child_1/></elt>
  <elt id="elt3"><child_1/><child>child 2</child></elt>
  <elt id="elt4"><child_1/></elt>
</doc>