Thanks for trying that out. Deparse is a good idea, I get the same results as you except for case two, where my Deparse says

say sort('get_keys' ());
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vs your

say((sort get_keys ()));
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Interestingly, feeding my Deparse output back to perl leads to a syntax error. Your version seems to indicate what perl is doing here. Trying to sort the empty list () with the compare-function get_keys, hence the empty return.