I wrote about Perl variable declaration secrets recently, but there's much more. Perl has a very simple way for passing variables to subs which should be called later.
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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.