Thank you so much for your informative and meaningful advice. So far your codes run very well, but if you don't mind, I'd like to ask you some questions about your codes:

1) You set twig_handlers to two key elements and called corresponding subroutines. What I couldn't understand is, as you didn't assign parameters in &topic and &extpage, what's the meaning of:
my ( $twig, $topic ) = @_;

2) About %links, the hash table you created:
Does the following codes mean you add each children link for att('re:resource')?
$links{ $_->att('r:resource') } = $_ for $topic->children('link');

3) About your two sub routines, I understand that at first, you walk through the whole xml document to find out the fist <topic/> node and if it has <link/> child, you save link information to hash table and then examine <ExternalPage/> followed it. If it doesn't have a <link/> child, you reset the hash to empty, here is my question, if the hash is empty, will you examine <ExternalPage/> as well coz I'm really doesn't know how these two subroutine connect with each other or what is the run order of them? Is it run one <Topic/> then all <ExternalPage/> or all <Topic/> first then All <ExternalPage/>?