Please use <code> tags to format your code fix your <code> tags, see How do I post a question effectively? and Markup in the Monastery.

But based on your problem description, here is a template that reads two CSV files of the same length in parallel and writes to a third (which you can then use to replace the second file, if you like) - I think this is what you're after? You can adjust the logic for creating the new rows to fit your needs. It uses Text::CSV, and you should also install Text::CSV_XS for speed.

use warnings;
use strict;
use Data::Dumper; # Debug
$Data::Dumper::Useqq=1;
use Text::CSV;

my $file1 = 'in1.txt';
my $file2 = 'in2.txt';
my $outfile = 'out.txt';

my $csv = Text::CSV->new({binary=>1, auto_diag=>2, eol=>$/});

open my $ifh1, '<', $file1 or die "$file1: $!";
open my $ifh2, '<', $file2 or die "$file2: $!";
open my $ofh, '>', $outfile or die "$outfile: $!";

$csv->getline($ifh1); # read and discard header
$csv->getline($ifh2); # read and discard header
$csv->print($ofh, ['col1','col2']); # new header

while ( my $row1 = $csv->getline($ifh1) ) {
    my $row2 = $csv->getline($ifh2);
    die "file1 has more lines than file2" unless $row2;
    
    # do whatever you like to create the output row here
    my $orow = [@$row1, @$row2]; # example: join the two rows
    
    print Dumper($row1, $row2, $orow); # Debug
    $csv->print($ofh, $orow);
}
die "file2 has more lines than file1" unless eof($ifh2);

close $ifh1;
close $ifh2;
close $ofh;

$csv->eof or $csv->error_diag;

[download]

Minor edits.

In reply to Re: Adding an array to an existing csv file as a new column by haukex
in thread Adding an array to an existing csv file as a new column by c.con

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