Move the POD away from $a and $b.

They are built ins and should not be used or overrided. Currently doing
pod/perlthrtut.pod .

For: RT #123081 (partial)
Move away from $a and $b for examples.

pod/perlvar.pod this time.

For: RT #123081 (partial)
$a,$b deprecation in perlop.pod.

For: RT #123081 (partial)