You can reduce the number of math ops and save some time. It won't make the Perl compete with the C, but you can apply the same optimization to the C, and make it that much faster. Basically, I'm factoring the multiplication out of the inner loop. The division can be done after all the looping. 
for my $i (0..$#data){
    my $sum_j = 0;
    $sum_j += $data[$_] for $i..$#data;
    $pears += $data[$i] * $sum_j;
}

[download]
This was about 3x faster than (my interpretation of) your original, for me. Some of that comes from changing the looping constructs, but a chunk of it comes from fewer math ops.
Caution: Contents may have been coded under pressure.

In reply to Re: PDL vs C speed question by Roy Johnson
in thread PDL vs C speed question by glwtta

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