See attached code. I used the map trick again: foreach ( map{$_-> [ 4]}@results) iterates over all of the contents of column 4 and a freq hash is built. A list of references to rows is what is going into the map. The map then de-references and transforms this such that the output is list of every contents of column 4.

The way sort works: <---output sort{...} <---input
is that what goes in is what comes out. What is coming in are references to rows of the @results array. What sort needs is a way to compare 2 rows: row A<row B, row A equal row B or row A>rowB. The function that provides the comparison can be anything that you want as long as it produces a consistent result (reverses the answer if a and b are reversed).

So I look up the value of col 4 for say row A, then I ask the frequency hash what the frequency is and I compare that result with a likewise computation for row B. In the case of a tie, I use an alphabetic comparison of row 0. Note that I reversed a and b to get highest frequency first while I am sorting on lowest column 0 first.

The way that the sort decider function is written may appear a bit strange, but it is just returning a: -1, 0 or 1 depending upon how row A and row B compare.

It is completely legal to assign the sorted result set back to the input variable and I did that. To get your printout, just do the column 4 look up in the freq hash to get frequency. The order of my @result jives with the order of your output.

For printing, of course you can access each element as a 2-D coordinate, but usually better is to iterate over the rows with row reference like this:

foreach my $row (@results)
{
   print "$row->[0]  $row->[1]\n";
}
[download]

I think the following code does what you want...

#!/usr/bin/perl

use warnings;
use strict;
use Data::Dumper;
use Data::Dump qw(pp);

my @results = (["chpt10_2", "sent. 2", "alice", "nsubj", "animals", "p
+rotect"],
               ["chpt12_1", "sent. 54", "bob", "nsubj", "cells", "prot
+ect"],
               ["chpt25_4", "sent. 47", "carol", "nsubj", "plants", "p
+rotect"],
               ["chpt34_1", "sent. 1", "dave", "nsubj", "cells", "prot
+ect"],
               ["chpt35_1", "sent. 2", "eli", "nsubj", "cells", "prote
+ct"],
               ["chpt38_1", "sent. 1", "fred", "nsubj", "animals", "pr
+otect"],
               ["chpt54_1", "sent. 1", "greg", "nsubj", "uticle", "pro
+tect"]
              );
my %freq;
foreach ( map{$_->[4]}@results) #feeds in list of animals, cells, utic
+le, etc.
{
   $freq{lc $_}++;
}


@results = sort {$freq{lc $b->[4]} <=> $freq{lc $a->[4]} #freq order
                                   or
                         $a->[0]  cmp $b->[0]            #text col 0  
+    
                } @results; 

print pp(\@results);


__END__

[
  ["chpt12_1", "sent. 54", "bob", "nsubj", "cells", "protect"],
  ["chpt34_1", "sent. 1", "dave", "nsubj", "cells", "protect"],
  ["chpt35_1", "sent. 2", "eli", "nsubj", "cells", "protect"],
  ["chpt10_2", "sent. 2", "alice", "nsubj", "animals", "protect"],
  ["chpt38_1", "sent. 1", "fred", "nsubj", "animals", "protect"],
  ["chpt25_4", "sent. 47", "carol", "nsubj", "plants", "protect"],
  ["chpt54_1", "sent. 1", "greg", "nsubj", "uticle", "protect"],
]
[download]