my $twig= XML::Twig->new( twig_handlers => { title => \&title })
                   ->parsefile( "foo.xml");

sub title
  { my( $twig, $title)= @_;
    # return 1 (true) if you can have other handlers on the
    # title, otherwise it doesn't matter what you return
    # see the doc for twig_handlers 
    return 1 unless( $title->root->matches( 'gutbook');
    $title->set_tag( 'tit');
  }