Perl 5 stuff I learned or relearned in 2019

[Eric Wong <e@80x24.org>]

It seems like I've learned a lot about Perl5 this year, which
is strange for something I've been using for so long.

Some good, some bad, but damn: Perl is a giant language.  2019
was also my first time using Perl 5.10 features (from 2007).

- the defined-or ("//") operator from 5.10 is awesome

- \w, \s, \d and their inverses match Unicode chars starting with 5.8;
  not just the traditional ASCII-only chars (I suspect potential
  security vulnerabilities, here)

- Perl 5.16 and earlier had several memory leaks (which could've
  affected other projects I've worked on..)

- `# line $NUM "FILE"` comments are interpreted by eval; like CPP

- PerlIO layers, we're using PerlIO::scalar.  This is also
  independent of the "tie" interface used by IO::Socket::SSL.
  Only "tie" seems suitable for non-blocking I/O, though.

- source filters (perlfilter(1), Filter::Simple).  One of the
  few things I like about Python is indentation, so Acme::Pythonic
  here I come! (ok, not for this project, at least :P)

- rediscovering the flexibility and power of "local" on
  globals like %SIG, %ENV, etc...

- the semi-colon to distinguish blocks from hashref literals
  for "map {;" calls and such (how did I not know that before? :x)

- the native (C) signal handler installed by sigaction does not use
  a self-pipe or eventfd; so it's susceptible to missed wakeups.
  However, POSIX.pm supports sigprocmask, so I can use signalfd
  or kqueue EVFILT_SIGNAL in my event loop depending on OS.  In
  retrospect, I like this hands off, worse-is-better approach of
  the interpreter.

- "fc" (fold-case) for Unicode-aware case comparisons
   (only in 5.16+, though)

- XS typemaps saves a lot of boring grunt work (other projects)

Anyways, maybe this helps anybody hacking or thinking about
hacking on this project.