This all comes back to the error message:

Type of arg 1 to keys must be hash or array (not list) at - line 3, ne
+ar "%Z)"
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keys() really wants a hash as its single argument. Not a list. And %A, %Z is a list. Admittedly, it is a list of pairs that can again be interpreted as a hash, but keys() wants to have that hash.

Using +{ %A, %Z }->%* or %{{ %A, %Z }} constructs the list of pairs from the two hashes, wraps these in a hash reference and then dereferences that hash reference, giving the hash to keys(). This is why that one works.

Your other approaches fail because keys() only takes a single argument, and that argument must be a hash. Everything you try to pass to it is a list, because the comma operator "," creates a list or the assignments go through an intermediate list (and don't return the assigned hash).

If you are only interested in getting a unique list of keys, you could use List::Utils uniq, but that is not shorter than the two step approach, especially if you also later want to look at the combined values:

my @all_keys = uniq( keys(%A), keys(%Z) );
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