How to extract multiple columns from a CSV file using Perl

I published a module called Tie::Array::CSV which lets Perl interact with your CSV as a native Perl nested array. If you use this, you can take your search logic and apply it just as if your data were already in an array of array-references. Take a look!

#!/usr/bin/env perl

use strict;
use warnings;

use File::Temp;
use Tie::Array::CSV;
use List::MoreUtils qw/first_index/;
use Data::Dumper;

# this builds a temporary file from DATA
# normally you would just make $file the filename
my $file = File::Temp->new;
print $file <DATA>;
#########

tie my @csv, 'Tie::Array::CSV', $file;

#find column from data in first row
my $colnum = first_index { /^\w.{6}$/ } @{$csv[0]};
print "Using column: $colnum\n";

#extract that column
my @column = map { $csv[$_][$colnum] } (0..$#csv);

#build a hash of repetitions
my %reps;
$reps{$_}++ for @column;

print Dumper \%reps;

Here's a sample script using the Text::CSV module to parse your csv data. Consult the documentation for the module to find the proper settings for your data.

#!/usr/bin/perl
use strict;
use warnings;
use Text::CSV;

my $csv = Text::CSV->new({ binary => 1 });

while (my $row = $csv->getline(*DATA)) {
    print "Date: $row->[2]\n";
    print "Col#1: $row->[3]\n";
    print "Col#2: $row->[4]\n";
}

You definitely want to use a CPAN library for parsing CSV, as you will never account for all the quirks of the format.

Please see: How can I parse quoted CSV in Perl with a regex?

Please see: How do I efficiently parse a CSV file in Perl?

However, here is a very naive and non-idiomatic solution for that particular string you provided:

use strict;
use warnings;

my $string = '"ID","URL","DATE","XXID","DATE-LONGFORMAT"';

my @words = ();
my $word = "";
my $quotec = '"';
my $quoted = 0;

foreach my $c (split //, $string)
{
  if ($quoted)
  {
    if ($c eq $quotec)
    {
      $quoted = 0;
      push @words, $word;
      $word = "";
    }
    else
    {
      $word .= $c;
    }
  }
  elsif ($c eq $quotec)
  {
    $quoted = 1;
  }
}

for (my $i = 0; $i < scalar @words; ++$i)
{
  print "column " . ($i + 1) . " = $words[$i]\n";
}