Some errors are really hard to find: They appear only sometimes or only on live systems or within complex source that can't run manually using a debugger. Adding debug output might help, but might also be confusing as the DBI error code 4 "statement contains no result" does.
Im ersten Teil habe ich den Taint mode vorgestellt und gezeigt, wie gefährlich unbekannte Daten sein können. Jetzt geht es um die nicht minder wichtige Frage, wie man sich diese Daten trotzdem nutzbar machen kann. Das Ziel einer jeden (Web-)Anwendung sollte es sein, mit legitimen Eingaben problemlos und effizient umzugehen ohne dabei die Sicherheit gegenüber Betrugsversuchen leiden zu lassen.
(SQL) Injection ist immer wieder für Bugs gut, vor allem bei Webanwendungen, aber auch andere Stellen können ganz leicht sehr gefährlich werden. PHP hat das lange Zeit in eindrucksvoller Weise demonstriert - dazu reichte schon ein sehr simples index.php:
Data::ObjectDriver is a great ORM. It's easy to configure and easy to use, but not as powerful as DBIx::Class (which isn't that easy to learn and I actually prefer using a wrapper instead of "native" DBIx::Class, but that's another story). There is one major thing I missed with Data::ObjectDriver: JOINing foreign tables.
Perl has a very flexible variable concept. One might use different namespaces, called "packages" or use all variables global without defining them. Another way to use variables is more Cish where every single variable must be declared before it's used.
Perl has a great asynchronous library: AnyEvent. (There may be even more great asynchronous libraries, but it decided to use AnyEvent.) I recently had to lookup a lot of different hostnames and didn't want to do it sequentially (because every single DNS server might be down or wait until the reply is received).
DKIM may be called as PGP successor: PGP has been used by many people for signing their emails at a time when mostly technical related people had been using the internet. Today, few people still use PGP to sign their emails, but email servers took over this part using a technology called DKIM.
Some websites stopped working today or at least some cookie-based functions ran out-of-order. Deleting all cookies helps - but is no solution. A cookie called __atrfs set by the famous AddThis! service is responsible for this.