use HTML::TokeParser::Simple;
my $p = HTML::TokeParser::Simple->new( \$original_html );

my $new_html = '';
while ( my $token = $p->get_token ) {
    unless ($token->is_text) {
        $new_html .= $token->return_text;
    }
    else {
        my $text = $token->return_text;
        $text =~ s/foo/<bar>foo</bar>/g;
        $new_html .= $text;
    }
}