However, when using my $x = qr/(??{ $x })/;, the script throws no strictures errors

That's the bug I mentioned earlier. The use strict; didn't propagate into the (??{ }). (You're getting the warning because the second $x is the package variable.)

I'm guessing the behaviour of our $x = qr/(??{ $x })/; changed when they fixed the pragma propagation.

I'll file a bug report.