%hash<b> is short for %hash{<b>} which is short for %hash{qw<b>}
There is also %hash«b» / %hash<<b>> which is similarly short for %hash{«b»} / %hash{qqww«b»}

With that knowledge it should be easy to see why %hash<$somekey> would be the same as %hash{'$somekey'}.

  use v6.c;

  my %hash = (
    :a<A>,
    :b<B>,
    'c d' => 'C~D',
    'a b' => 'A~B',
    '$somekey' => '?'
  );

  my $somekey = 'a b';

  say %hash«  $somekey  'c d' ».perl; # ("A", "B", "C~D")
  say %hash« '$somekey'       ».perl; # "?"
  say %hash« "$somekey"       ».perl; # "A~B"

  say %hash<  $somekey        >.perl; # "?"
[download]