Even faster: 
tin4=> sub {
  my $b = 0;
  $output[$_] = $arr[$b++] + $arr[$b++] + $arr[$b++]
    for 0..@arr/3;
},
tin5=> sub {
  my $b = -1;
  $output[$_] = $arr[++$b] + $arr[++$b] + $arr[++$b]
    for 0..@arr/3;
},

[download]
       Rate tin1 tin2 tin3 tin4 tin5
tin1 10.2/s   -- -54% -59% -65% -68%
tin2 22.2/s 118%   --  -9% -23% -30%
tin3 24.5/s 141%  10%   -- -15% -23%
tin4 28.8/s 183%  30%  17%   --  -9%
tin5 31.7/s 211%  43%  29%  10%   --

[download]

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

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