Another way ... :-)

#!/usr/bin/perl -w

  use strict;
  use warnings;

  my $filename = 'library.xml';
  
  use XML::LibXML;

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

  
  print "\nBooks\n";
  my @nodes = $doc->findnodes("/library/book[last()]"); 
  foreach my $node (@nodes) {
    print $node->findvalue( 'title')."\n";
    my @authors = $node->findnodes('author');
    print "Authors:".scalar(@authors)."\n";
    foreach my $auth (@authors) {
        print $auth->textContent()."\n";
    }

}
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