%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; # "?"