use HTML::TokeParser::Simple 2.1;
my $parser = HTML::TokeParser::Simple->new( $ugly_html );
while (my $token = $parser->get_token) {
    rewrite_body_tag($token) if $token->is_start_tag('body');
    $token->rewrite_tag;
    print $token->as_is;
}

sub rewrite_body_tag {
    # what could be easier?
    my $token = shift;
    $token
        ->delete_attr('bgcolor')
        ->delete_attr('alink')
        ->set_attr('class','foobar');
}