In Perl 5 my $var; ... for $var (...) {...} doesn't alter the contents of the $var global to the for loop. Is that true for both the -> and <-> variants of the Perl 6 for loop illustrated above?

Yes.

presume you can for @array <-> my $var {...}, in Perl 6 to make the scope of $var obvious.

No, you can't. Both arrows actually introduce a signature, and my in a signature doesn't make sense.

I used to write -> my $x too, but got used to omitting the my rather quickly :-).