Is 
$output[int($_/3)] += $arr[$_] for 0..@arr-1;

[download]
efficient enough, or is there something I am missing? Too many "int()"s? My guess is that 
my $k = @arr / 3;
for (0..$k) {
    $output[$_] += $arr[3*$_] + $arr[3*$_+1] + $arr[3*$_ +2];
}

[download]
is about as efficient as you'd get.
Edit
Actually, it seems to me that 
my $k = @arr / 3;
my $b = 0;
for (0..$k) {
    $output[$_] = $arr[$b] + $arr[$b+1] + $arr[$b +2];
    $b += 3;
}

[download]
is probably a tad faster. Although if your problem really is as big as to make a difference between those 2 versions, then you shouldn't really be using Perl.

In reply to Re: Summing Up Array Elements By Group by ivancho
in thread Summing Up Array Elements By Group by neversaint

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