Oh I was sure the syntax was ok in both forms. I'm guessing it's a GLR revision. It's indeed better to to reserve {} for scalar containers.

Notice how in Perl 6 you get a nice error message:

    $ perl6 -e 'my %h = { foo => "bar" };'
    Potential difficulties:
        Useless use of hash composer on right side of hash assignment;
        did you mean := instead?
        at -e:1
        ------> my %h = { foo => "bar" }⏏;