The first one you wrote was the fastest: 
Benchmark: timing 1000000 iterations of tin1, tin2, tin3...
      tin1:  1 wallclock secs ( 0.59 usr +  0.00 sys =  0.59 CPU) @ 16
+86340.64/s (n=1000000)
      tin2:  1 wallclock secs ( 0.75 usr +  0.00 sys =  0.75 CPU) @ 13
+33333.33/s (n=1000000)
      tin3:  3 wallclock secs ( 2.22 usr +  0.00 sys =  2.22 CPU) @ 45
+0450.45/s (n=1000000)
          Rate tin3 tin2 tin1
tin3  450450/s   -- -66% -73%
tin2 1333333/s 196%   -- -21%
tin1 1686341/s 274%  26%   --

[download]
the input to get this was: 
$results = timethese(1000000,{
tin1 => '$output[int($_/3)] += $arr[$_] for 0..@arr-1;',
tin2 =>'$out[$_] = ($arr[(3*$_)]+$arr[(3*$_+1)]+$arr[(3*$_+2)]) for 0.
+.(scalar(@arr)/3 - 1);',
tin3=>'for (0..$k) {
    $output[$_] = $arr[$b] + $arr[$b+1] + $arr[$b +2];
    $b += 3;
}
'});
Benchmark::cmpthese( $results );

[download]
Raghu

In reply to Re^2: Summing Up Array Elements By Group by imrags
in thread Summing Up Array Elements By Group by neversaint

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