I like XPath and found some XPath solutions to the problem on stackoverflow here including this one: //*[not(child::*)]. XML::Twig didn't seem to handle the XPath expressions on stackoverflow but XML::XPath did. XML::XPath also comes with an 'xpath' grep like utility that might give you a solution as simple as:

xpath -e '//*[not(child::*)]' books.xml
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If you want to do Perl coding for further processing the solution is still pretty simple

use strict;
use warnings;

use XML::XPath;
use XML::XPath::XMLParser;
    
my $xp = XML::XPath->new(filename => 'books.xml');
    
my $nodeset = $xp->find('//*[not(child::*)]');
    
foreach my $node ($nodeset->get_nodelist) {
    print "FOUND:", 
        XML::XPath::XMLParser::as_string($node),
        "\n";
}
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