I think this is a bug in Perl, but perhaps it should be one of those "don't do that" sort of bugs.

Colour coding helps, in this case. To see why the one that doesn't compile doesn't compile, paste both at http://tnx.nl/scribble.plp.

It is not a bug, but Perl 6 will fix it nevertheless. See also "Gory details of parsing quoted construct" in perlop, which explains that "Finding the end" is done before "Interpolation".