The version of XML::LibXML that's in RHEL5 (1.58) performs badly when performing findnodes on the results of findnodes for large XML files. The latest version from CPAN performs fine.

  use XML::LibXML;

  my $parser = XML::LibXML->new();
  my $doc    = $parser->parse_file($filename);

  foreach my $book ($doc->findnodes('/library/book')) {
    my($title) = $book->findnodes('./title');
    print $title->to_literal, "\n" 
  }
[download]

Here is my workaround for RHEL5:

  use XML::LibXML;

  my $parser = XML::LibXML->new();
  my $doc    = $parser->parse_file($filename);

  foreach my $book ($doc->findnodes('/library/book')) {
    my($title) = $book->getChildrenByTagName('title');
    print $title->to_literal, "\n" 
  }
[download]

Despite $book->childNodes returning only $book's child nodes, $book->findnodes appears to perform findnodes on the entire contents of $doc.