[Perl/perl5] d96b4e: 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: d96b4ed5954b4baf2632159a91b8c6a0f219b51d
      
https://github.com/Perl/perl5/commit/d96b4ed5954b4baf2632159a91b8c6a0f219b51d
  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/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

  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`.

I dind't touch `perlfilter.pod`, since that comes from the
Filter dist, which is upstream CPAN.