use strict;
use warnings;
use XML::LibXML;

use XML::LibXML qw( );

my $parser = XML::LibXML->new();
my $doc = $parser->parse_file('out.xml');

for my $result ($doc->findnodes('/books/book')) {
for my $interv ($result->findnodes('book')) {
   my $bk  = $interv->find('code_a');
   my $bk_name = $interv->find('name');

   print("$bk - $bk_name");
}
}