Hi, thanks a lot for your answer, I'll have a look at Text::CSV XS. I must admit this is slightly over my head, I've replaced $previous_line = $current_line with: 

$current_columns[2] += $previous_columns[2];
$previous_line = join ' ', @current_columns;

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This cumulates the 3rd column, which although I mentioned it in the question on reflection isn't actually what I was wanting to do. Apologies for wasting your time with a badly worded question. I'm looking to alter a value in the current line, and then have that value saved to the previous line for use in the next iteration of the loop.

In reply to Re^2: How to loop over two lines, alter a value in the current line and save it to the previous line? by rjc33
in thread How to loop over two lines, alter a value in the current line and save it to the previous line? by rjc33

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