This is not such much a comment about the possible perl6 optimizations, but on the meaning of the perl6 code you gave.

My understanding of perl6 is that it <H> gobbles as much and no more then the assignement context warrants.
($a, $b) = <STDIN>
eats two lines of std input. But I suppose that
print <STDIN>
eats all of STDIN. So you example would be optimized to the perl5 equivalent of
print scalar  <STDIN>;

Please correct me if I am wrong.