English.pm: update perl version where perf fixed

It still said that the performance of $`, $&, $' was fixed in 5.18.
Update that to 5.20, since COW wasn't enabled by default in 5.18.
perlvar.pod: add a separate section on $& et al

Add a new separate section explaining the performance issues of $`, $&
and $'; plus descriptions of the various workarounds like @-, /p and COW,
and which perl version they were each introduced in.

Then in the entries for each individual var, strip out any commentary
about performance, and just include a link to the new performance
section.