my %is_prime;
   @primes = grep { 
                ( exists $is_prime{$_} and $is_prime{$_} )
                or 
                ( $is_prime{$_} = is_prime($_) )
             } @numbers;
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Do I understand correctly that for numbers where $is_prime{$_} is false, the cached value is overwritten by a (supposedly superfluous) evaluation of is_prime($_)?