[perl #70764] $' fails to initialized for pre-compiled regular expression matches

[perl #70764] $' fails to initialized for pre-compiled regular expression matches

The match vars are associated with the regexp that last matched
successfully. In the case of $str =~ $qr or /$qr/, since the $qr could
be used in multiple scopes that need their own sets of match vars, the
$qr is cloned by Perl_reg_temp_copy as of change 30677/28d8d7f. This
happens in pp_regcomp before pp_match has stringified the LHS, hence the
bug. In short, /$gror/ is not equivalent to
($which = !$which) ? /$gror/ : /$gror/, which is weird.

Attached is a patch, which admittedly is a hack, but fixes this
particular side effect of what is probably a bad design, by stringifying
the LHS in pp_regcomp, and having pp_match skip get-magic in such cases.
A real fix far exceeds my capabalities, and would also be very intrusive
according to
<http://www.nntp.perl.org/group/perl.perl5.porters/2007/03/msg122415.html>.