[Perl/perl5] 81b768: 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: 81b768e4d2df7068c6cc007cab36704785056058
      
https://github.com/Perl/perl5/commit/81b768e4d2df7068c6cc007cab36704785056058
  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.