#!/usr/bin/env perl
use strict;
use warnings;
use HTML::TreeBuilder;
use Data::Dumper; $Data::Dumper::Sortkeys = 1;

my $tree = HTML::TreeBuilder->new;
$tree->parse( <<'END_OF_HTML' );
    <span> Author_name </span>
    __filler__
    <ul>
      <li> book 1 by Author_name </li>
      <li> book 2 by Author_name </li>
    </ul>
    <span> New_Author </span>
    __filler__
    <ul>
      <li> book 1 by new </li>
    </ul>
END_OF_HTML
$tree->eof;

# Uncomment to show that as_HTML is a bad fit for this task.
# open my $fh , '<', \( $tree->as_HTML('', '  ') ) or die;
# print $_ while <$fh>;
# exit;

my @tags = $tree->find_by_tag_name( qw( span li ) );

my $current_author;
my %book_author;
my %author_books_HoA;
for my $t (@tags) {
    my $tag_name = $t->tag;
    if ( $tag_name eq 'span' ) {
        $current_author = $t->as_trimmed_text;
    }
    elsif ( $tag_name eq 'li' ) {
        next unless $t->parent->tag eq 'ul';

        my $book_title = $t->as_trimmed_text;

        warn if exists $book_author{$book_title};
        $book_author{$book_title} = $current_author;

        push @{ $author_books_HoA{$current_author} }, $book_title;
    }
    else {
        die "Unexpected tag $tag_name"
    }
}

print Dumper \%book_author, \%author_books_HoA;