Pod::Simple::BlackBox.pm: Fix for early perls

A previous commit would break this module for Perls earlier than v5.6,
as that is when /[[:ascii:]]/ was introduced.  This patch adds an
alternative path for such early Perls that works completely for ASCII
platforms, and almost completely for EBCDIC.

Feel free to merge this with the earlier patch
Pod::Simple::Simple.pm: Use $^V not $]

We were talking about this on p5p today, and it's better to use $^V if
feasible, and it is here.  There is another occurrence of $] in the
file, but I didn't change it.

Feel free to ignore this patch or merge it with the earlier one.