See http://perl101.org/hashes.html, for example. (There are many other webpages that one can visit to get similar tutorials. This one was #3 in the list of search results when I Googled "hashes perl".)

Hashes. They are simple. I give it a key, I get back a value. Very fast.

I have been testing some code on a smaller scale and something is confusing me. Why does the  print "$csv2\n"; command at the end of this script print each full line of the csv. $csv2 has been populated with the key data of csv2hash, so I'm confused as to why it would print each line? Is each line considered a key in this instance?

#!/bin/perl


 my @csv2 = (); # Creates new empty array @csv2

      open CSV2, "<csv2" or die; # Creates the file handle CSV2, input
+s the data from csv2 into CVS2

      @csv2=<CSV2>; # populates @csv2 array with data from file handle
+ CSV2
      close CSV2;

      my %csv2hash = ();  # Creates empty hash csv2hash

      for (@csv2) {            # for data in @csv2 array
      #chomp;

       my ($title) = $_ =~ /^.+?,\s*([^,]+?),/; #/define the data whic
+h is the title
#Indicate that title data will input into csv2hash
        $csv2hash{$_} = $title;

}

foreach my $csv2 (keys %csv2hash) { # iterates over all the keys in %c
+sv2hash, assigning each one to $csv2



    my $value = $csv2hash{$csv2};



print "$csv2\n"; # This prints the data line by line in the csv2
[download]

The keys of %csv2hash are the lines:

for (@csv2) {            # for data in @csv2 array
      my ($title) = $_ =~ /^.+?,\s*([^,]+?),/;
      $csv2hash{$_} = $title;
}
[download]

Here, $_ (the current line) is used as a key into %csv2hash.