I know that you already have a solution. I believe that a one-pass solution with fewer temporary variables is easier to understand and maintain. 
# copy headers to output
my $headerA = <$dataA> . <$dataA>;
my $headerB = <$dataB> . <$dataB>;
print $dataC "__dataC__\n$headerA";

while (1) {
    last if (eof($dataA) or eof($dataB));
    my @A = (split( /\s+/, <$dataA>))[0..3];
    my @B = (split( /\s+/, <$dataB>))[0..3];
    my $avgX = ($A[0] + $B[0])/2;
    my $avgY = ($A[1] + $B[1])/2;
    my $avgZ = ($A[2] + $B[2])/2;
    my $difF = ($A[3] - $B[3]);
    printf $dataC "%E %E %E %E\n", $avgX, $avgY, $avgZ, $difF;
}

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Bill

In reply to Re: combining two csv files by using math operations by BillKSmith
in thread combining two csv files by using math operations by ng0177

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