Merge branch 'hugmeir/disallow_cntrl' into blead

[perl #119123] disallow literal control character variables

This branch introduces a deprecation warning on variables like $^T,
where ^T is a literal control character in the source code.

It also reworks how the handling of whitespace in ${...} works,
fixing a number of bugs, such as ${\nfoo\n} not increasing the
line number.