I don't know if that worked before, but I do find the error message okay, as

(map { $_ => 1 } @array)
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returns a list, not an array, which "keys" would also accept as a valid argument.

You get the same error if you try:

my @unique = keys (1,1,2,3,2,2);
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but not with:

my @unique = keys @array;
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which returns the indices of the array.