Definitely involves mean and stddev. Hopefully the snippet shows how I'm trying to do this, but do let me know if there is another way, thanks.

    foreach my $subrow (keys %pair1data) {
        $zee1 = ( ( $pair1data{$subrow} - $submean1 ) / $substddev1 );
        $zee2 = ( ( $pair2data{$subrow} - $submean2 ) / $substddev2 );
        $sigma += ( $zee1 * $zee2 );
        $datasetcounter ++;
    }
    $r = ( $sigma / ( $datasetcounter - 1 ) );
    return $r;
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