[Perl/perl5] c46e7e: pod: eliminate references to `$a` and `$b` outside...

[Dagfinn Ilmari Mannsåker via perl5-changes]

  Branch: refs/heads/pod-dollar-ab
  Home:   https://github.com/Perl/perl5
  Commit: c46e7e5595fe98dbab49dee4cdf6a3c719b48bbe
      
https://github.com/Perl/perl5/commit/c46e7e5595fe98dbab49dee4cdf6a3c719b48bbe
  Author: Dagfinn Ilmari Mannsåker <ilm...@ilmari.org>
  Date:   2023-08-16 (Wed, 16 Aug 2023)

  Changed paths:
    M pod/buildtoc
    M pod/perlcall.pod
    M pod/perldbmfilter.pod
    M pod/perldiag.pod
    M pod/perldsc.pod
    M pod/perlebcdic.pod
    M pod/perlembed.pod
    M pod/perlfilter.pod
    M pod/perlfunc.pod
    M pod/perlgit.pod
    M pod/perlhacktips.pod
    M pod/perlinterp.pod
    M pod/perlintro.pod
    M pod/perllocale.pod
    M pod/perlop.pod
    M pod/perlpodspec.pod
    M pod/perlre.pod
    M pod/perlrebackslash.pod
    M pod/perlrecharclass.pod
    M pod/perlref.pod
    M pod/perlsyn.pod
    M pod/perltie.pod
    M pod/perluniintro.pod
    M t/porting/customized.dat

  Log Message:
  -----------
  pod: eliminate references to `$a` and `$b` outside sort context

This causes confusion for new users when they get told to not use
those variables because of how they interact with sort and friends.

This changes them to `$x` and `$y`, and also changes nearby variables
to keep things consistent, such as `$c` -> `$z` and `@a` -> `%x`.