The problem is that, in the argument list of a sub or method, any key => 'value' pair with an unquoted key is always treated as a named argument. So any such argument it is always routed into the slurpy hash, and never into the slurpy array.

The simplest solution is to quote the key of each pair, in which case they will be treated as positional arguments and will be routed into the slurpy array.

Of course, each such positional pair will be a single Pair object (not a flattened list of key, value) so you will end up with an array of pairs, not an array of alternating keys and values. That means you'll need to unpack them differently in the for loop.

Putting all that together, we get:

class testing {
    method newhash(*%args) {
        for %args.kv -> $k, $v {
            say "Hash Key $k => Value $v";
        }
    }
    method newarray(*@args) {
        for @args».kv -> [$k, $v] {
            say "Array Key $k => Value $v";
        }
    }
}    

my $x = testing.newhash(a => 1, b => 2);      # Messes with specified 
+order

my $y = testing.newarray('c' => 3, 'd' => 4);   # Looks okay, and work
+s
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If you simply must have unquoted keys, then you'll need to pass an explicit list of them (in parens), because the "unquoted-means-named" magic only applies at the outermost level (known as the "comma level") of the argument list. So, with the rewritten newarray method above, the following would also work:

my $y = testing.newarray((c => 3, d => 4));   # Looks okay, and works
[download]

It seems that you can't do that other than sorting keys:

class testing {
    method newhash(*%args) {
        for %args.sort(*.key)>>.kv -> ($k, $v) {
               say "Hash Key $k => Value $v";
        }
        %args.perl.say;
    }
}

my $x = testing.newhash(a => 1, b => 2, c => 3, d => 4);
[download]

Here's what docs.perl6.org says about it:

Note that the order of the keys and values printed cannot be relied upon; the elements of a hash are not always stored the same way in memory for different runs of the same program. Sometimes one wishes to process the elements sorted on, e.g. the keys of the hash.

There you can also find canonical example.

The code %args.perl.say also do keys sorting, no magic there either.

It seems to me you have only one way to put your keys in order - name them in sortable manner and sort it later.