while ($body_str =~ /"http:\/\/([^ \/"]+)[^ "]*"/g) {
    if ($1 ne 'www.danheller.com') {
        $pos = pos($body_str);
        substr($body_str, $pos - 1, 1) = '" target="blank"';
        pos($body_str) = $pos;
    }
}
[download]

The regex might need a bit of upgrading, but it should work fine if your pages don't have sloppy HTML syntax (leaving out one or both ").