but there also is the non-looped version:

Except that x is a loop operator, just like map. In fact, it not only loops for the number of duplications, it also implicitely loops over each element of the list on the LHS.

@saw{@in} and keys(%saw) are also loops, but they are implicit unlike x.

In fact, you can remove two of the four loops of your "non-looped version":

undef %saw;
undef(@saw{@in});
@out = keys(%saw);
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Update: undef %saw is also an implicit loop, on calls other than the first.