This sounds like a job for those perl internals diagrams someone did at some point... Wish my memory was better.
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Mike

Yes. Instead of one scalar of the value '1' for every key, you get to share the same undef value for all the keys and thus don't have to allocate tons of memory you aren't going to use anyway. So keys @{{map { $_, undef } @ary}} has the potential to be a lot cheaper than keys @{{map { $_, 1 } @ary}}. The "a lot" part is just related to how many values are in your array. You burn one extra value for every entry in @ary and I'm just suggesting that you don't have to do that in this case.