I think "$ x" should be "\$ x". I have no problem with double-quote interpolation being more strict than the equivalent Perl code (it already is and this one more case would be an improvement).

I think it is just a matter of the original implementor(s) (way back in Perl2) deciding to support ignoring such whitespace (to match how Perl works outside of double quotes) and the more recent implementor(s) (p5p) not even thinking of supporting such a strange construct (and so not worrying about making them consistent).