foreach my $file (@files){
# set up the XML parser:
 my $twig= XML::Twig->new( 
  twig_handlers => { topicref => \&topicref_processing,
  		     xref => \&topicref_processing },
 );
 $twig->parsefile($file);
 $twig->purge;
}

##</code><code>##

sub topicref_processing {
  my($twig, $topicref) = @_;
  my($atts) = $topicref->atts();
  if($$atts{'keyref'}){
   ### associate this keyref with $file
  }
}