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

# global, but could be attached to the parser or passed to the handlers
my @results;

# we need those to hold info about the parsing
my( $elt_id, $child_seen, $in_child, $child_text);  

my $p= new XML::Parser( Handlers => 
                          { Start => \&start, # called when a start tag is found
                            End   => \&end,   # called when an end tag is found
                            Char  => \&char,  # called when characters are found
                          },
                       );
$p->parse( \*DATA);                           # use parsefile to parse... a file

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


sub start
  { my( $p, $elt, %atts)= @_;                 # set by XML::Parser::Expat

    if( $elt eq 'elt')                        # we found an elt start tag
      { $child_seen= 0;                       # reset the flag, no child found yet
        $elt_id= $atts{id};                   # store it in case we need it
      }
    elsif( $elt eq 'child')                   # found a child start tag
      { $child_seen= 1;                       # we've seen a child
        $in_child= 1;                         # we are in the child
        $child_text= '';                      # reset the child text
      }
  }

sub end
  { my( $p, $elt)= @_;
    if( $elt eq 'elt')                        # found and elt end tag
      { if( $child_seen)                      
          { push @results, $child_text; }     
        else
          { push @results, "missing child for elt $elt_id"; }
      }
    elsif( $elt eq 'child')                   # found a child end tag
      { $in_child= 0; }                       # Toto, I guess we are not in the child any more
  }

sub char                                      # called for all non mark-up text
  { my( $p, $string)= @_;
    $child_text .= $string if( $in_child);    # see the docs for why you can't
  }                                           # just write $child_text = $string

__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>