perlsyn says:

It is always commutative, i.e. $a ~~ $b behaves the same as $b ~~ $a .

Yes, it won't be commutative in a future release. At least not in general. Some matches will work regardless of the order, but not all.

Yes, Perl 6 gave whiplash to Perl 5 here; originally it was defined symmetrically, but we broke that after we realized it was silly to define a switch that was impossible to optimize to a jump table even if all the "when" values are small integers. The right side of a smart match is now always considered a pattern that makes its own decision how to match the left side, and it might or might not pay attention to the type of the left side to do so. In Perl 6, integer patterns always do integer comparison, and string patterns always do string comparison, regardless of the structure of the left side.