I think the best technique depends on the size of your jokes hash. If you have lots of jokes, use a separate file with the technique grinder describes (that works very well for randomly selecting a line from a file). If there are just some tens of jokes, then

$joke = $knockknocks{(keys %knockknocks)[rand keys %knockknocks]};
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works just as well, IMHO. Keep the easy things easy, and all that :-)