sub.t: To-do test for recursive shared-hash-keys TARGs

sub.t: To-do test for recursive shared-hash-keys TARGs

This is only buggy under ithreads.

sub a {
  for (${\""}.${\""}) {
    $_ = $_[0] || __PACKAGE__;
    print "$_\n";
    a("road") unless $_[0];
    print "$_\n";
  }
}
a();

The outer call sets the scalar returned by ${\""}.${\""} to the cur-
rent package name.

The inner call sets it to "road".

Each call prints it twice, the outer call surrounding the inner call.
The output in 5.10-5.18 is:

main
road
road
road

because the inner call is clobbering the same scalar.  If __PACKAGE__
is changed to "main", it works, and prints

main
road
road
main

(as the script above also prints in 5.8.8).