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           Rate      OP   other  fishbot   Joost
OP        293/s      --    -21%     -76%    -92%
other     369/s     26%      --     -69%    -90%
fishbot  1205/s    312%    226%       --    -68%
Joost    3723/s   1173%    908%     209%      --

use strict;
use warnings;
...
      OP       => 'scalar grep { $a{$_} !~ /^$/ } keys %a',
   }
);