I still think its the former.
Given that

%data = << why dunno who them where there which this >>;
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I would assume that writing *%data in any context is equivalent to writing

(why => "dunno", who => "them", where => "there", which => "this")
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So assuming we can use this as an lvalue, an assignment with a list of pairs to this would be the same as taking each pair and assigning it to the value with the specific key. So *%data = *%values should be compiled as

multi sub infix:=(List of Pair @assign is rw, List of Pair @values) re
+turns List of Pair {
  my %assign := *@assign;
  for @values {
     %assign{.key} = .value;
  }
  return *%assign;
}
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Let me take another example and go back to part(). We could write:

my $fish = "hest";
($sheep, $goats, $fish) := part Animal::Cat, @animals;
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So the question here is, is $fish eq "hest" after this, or undef?
Alternatly we could have written

my $hest = "hest";
%result := ($sheep, $goats, fish => $hest) := part Animal::Cat, @anima
+ls;
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Would that give the same result?

Update: Forgot the multi keyword
Update: As broquaint pointed out below, I forgot to use the binding operator as I indented to...
Update: And the second example is completly bogus, because it would endeed result in a compile time error. Was

($sheep, $goats, $fish => "hest") := part Animal::Cat, @animals;
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