Thanks, It looks like it's exactly what I'm looking for.

It's probably a good idea to use the Text::CSV_XS module to parse and recombine your csv files. This easily lets you get them into convenient array form.

Then, I'd store the arrays in a hash based on the first key. If the hash already has an array assigned to it, you can append to the existing array. (Or whatever you need to do. It's not exceedingly clear from the question. But this should be a good way to group them together.)

At the end, you can do a foreach on the keys of the hash, and combine the fields using the module, and write that out to a new csv file.

Hope this points you in the right direction!

Thanks, Now I got some good ideas on how to get it done properly.

use 5.006;  # Uses "open" syntax introduced in Perl 5.6.0.
use strict;
use warnings;

my $output_file_name = shift(@ARGV);
my $main_file_name   = shift(@ARGV);
my @other_file_names = @ARGV;

my @data;

foreach (@other_file_names) {
   open(my $fh_in, '<', $_)
      or die("Unable to open input file $_: $!\n");

   while (my $line = <$fh_in>) {
      # Keep the \n on $line.
      chomp(my $chomped_line = $line);
      my @fields = split(/,/, $chomped_line);
      foreach my $idx (0..$#fields) {
         # Saves a reference instead of a copy to save memory.
         push(@{$data[$idx]{$fields[$idx]}}, \$line);
      }
   }
}

{
   open(my $fh_in, '<', $main_file_name)
      or die("Unable to open input file $main_file_name: $!\n");

   open(my $fh_out, '>', $output_file_name)
      or die("Unable to open output file $output_file_name: $!\n");

   while (my $line = <$fh_in>) {
      print $fh_out ($line);
      chomp($line);
      my @fields = split(/,/, $line);
      foreach my $idx (0..$#fields) {
         if ($data[$idx]{$fields[$idx]}) {
            foreach (@{$data[$idx]{$fields[$idx]}}) {
               print $fh_out ($$_);
            }
         }
      }
   }
}
[download]

Untested. You didn't specify what to do And you should probably use Text::CSV_XS instead of splitting yourself.

Update: Fixed compilation errors.

Well the main file can be from 50 lines to anywhere 2000 lines.

That's tiny. But I don't load the main file into memory, only the other ones. I asked how big are the *other* files. If they're in the same range, you should have no problem.

If they all do have a field list there are a number of modules you can use that deal with CSV files. I'd recommend Text::xSV

Yes, each file has the first column of the first file, however the other files can have multiple instances of it. eg: 1st file: perl book,writer:xxxx,publisher:xxxx 2nd file: perl book,first edition,$29.99 2nd file: perl book,second edition,$25.99 3rd file: perl book,third edition,$x big file: perl book,writer:xxxx,publisher:xxxx||first edition,$29.99||second edition,$25.99||third edition,$x

This is almost good, however I do not want repeated lines. so if file B has multiple instances of Column 1 of file A, I want it just simply printed continuously, no line breaks. However I will still have to stick to Perl, because this will be an adon to a another script.

Next open the big CSV-file again with DBD::CSV and using an SQL statement such as "SELECT * FROM big.csv WHERE keyfield = key_value" replacing key_value by the keys in your hash, you extract the records one by one based upon the value of the keys.

As soon as you extract a record you write it to disk with Text::CSV_XS in another file.

If your csv-files are not too big, you could also try to read each record in each file with Text::CSV_XS and build a hash of arrays keyed by the value of your key-field and then empty the HoA back into a final CSV-file. This is probably faster but less intuitive.