You could say the parens are mostly ignored, here (they just influence operator precedence — between only commas, so no effect there), not induce grouping. Perl simply flattens the list. So your first example is equivalent to

my %hash=(
        "outer"=>
        "cool"=>1,
        "Kewl"=> 2,"odd"
);
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which is a fancy way of writing

my %hash=(
  "outer", "cool", 1, "Kewl", 2,"odd"
);
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You can witness this flattening of the list by passing your construct as parameters to a sub and have it check what it received:

use Data::Dumper;
sub test {
    print Dumper \@_;
}

test(
        "outer"=>(
            "cool"=>1,
            "Kewl"=>2,"odd"
        )
);
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Result:

$VAR1 = [
          'outer',
          'cool',
          1,
          'Kewl',
          2,
          'odd'
        ];
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