use HTML::Parser;
use Data::Dumper;

my $tag_string = 'word1 <tag0> word2 <tag1>word3 word4</tag1> word5 </tag0> word6 <tag2>word7 word8</tag2> word9 <tag3>word10</tag3> word11';

my $tags = parse_string( $tag_string );
print Dumper( $tags );

sub parse_string {
	my $string = shift;

	my %tags;
	my %check_tags;
	my $in_tag = 0;
	my $current_tag = 'no_tag';
	my $prev_tag;
	
	# process start tag event
	my $start = sub {
		my ($tag) = @_;
		$check_tags{$tag}++;
		$in_tag++;
		$prev_tag = $current_tag;
		$current_tag = $tag;
	};

	# process text event
	my $text = sub {
		my ($text) = @_;
		$text =~ s/^\s+//;
		$text =~ s/\s+$//;
		return if not length $text;
		my @words = split(m/\s+/, $text);
		push( @{ $tags{$current_tag} }, @words);
	};
	
	# process end tag event
	my $end = sub {
		my ($tag) = @_;
		$check_tags{$tag}++;
		$in_tag--;
		$current_tag = $in_tag ? $prev_tag : 'no_tag';
	};

	my $parser = HTML::Parser->new(
		api_version => 3,
		start_h     => [$start, "tagname"],
		text_h      => [$text,  "text"],
		end_h       => [$end,   "tagname"],
		default_h   => [$text,  "text"],
	);

	$parser->parse($string);
	$parser->eof;
	
	# check each tag has an end tag
	for my $tag (keys %check_tags) {
		if ($check_tags{$tag} % 2) {
			print "<$tag> is not valid\n";
		}
	}

	return \%tags;
}


##</code><code>##

$VAR1 = {
	'no_tag' => [
		'word1',
		'word6',
		'word9',
		'word11'
	],
	'tag0' => [
		'word2',
		'word5'
	],
	'tag1' => [
		'word3',
		'word4'
	],
	'tag2' => [
		'word7',
		'word8'
	],
	'tag3' => [
		'word10'
	]
};