I think what's confusing people here is the PDL notation - my fault, I should've given the pure Perl version - since $data[$i] and $data[$j] are PDL vectors, the '*' operator is overloaded to return a vector of the same length as its arguments, with every component being the product of the corresponding components of the input vectors. The sum() function simply returns the sum of the vector components.

The Perl version of my C example would look like this, assuming @data is an array of arrayrefs, rather than an array of PDL vectors:

for ($i = 0; i < @data; $i++) {
  for($j = $i + 1; $j < @data; $j++) {
    my $sum = 0;
    $sum += $data[$i][$_] * $data[$j][$_] for(0..@{$data[$i]}-1);
    my $pears = $sum / @{$data[$i]};
  }
}
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minor edit: forgot that square brackets get interpreted