Sure. uniq returns a list of unique elements, and lists evaluate to their number of elements in scalar context, so if you replace the line inside the foreach with e.g. this:

    say "$_: ", scalar uniq @{ $HoA{$_} };
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Then you'll get the number of unique items. (You could equally well assign to a variable for further processing, of course, and if you assign to a scalar, the explicit scalar won't be necessary, though it also won't hurt and may serve to make it a bit clearer what's going on.)