#!/usr/bin/perl

use Data::Dumper;
use strict;
my $k; 
my %hash; 
my @line;

# iterate thru data and store values in hash
while(<DATA>) {
    chomp($_);
    @line = split(",", $_);
    push @{ $hash{$line[2]} }, $line[4];
}
# hash structure (in case you are curious)
print "hash structure:\n";
print Dumper(\%hash);
print "\n";
# iterate thru hash
# send arrays to greatest sub
print "greatest values for each key:\n";
foreach $k (keys %hash) {
    print "$k: " . &greatest( $hash{$k} ) . "\n";
}

#return greatest value
sub greatest() {
    my $arr = shift;
    my @a = sort { $b <=> $a } @{ $arr };
    return $a[0];
}

exit;

__DATA__
2004,21,21,1,16.00
2004,21,21,1,16.25
2004,22,22,1,18.00
2004,22,22,1,18.25

##</code><code>##

hash structure:
$VAR1 = {
          '22' => [
                    '18.00',
                    '18.25'
                  ],
          '21' => [
                    '16.00',
                    '16.25'
                  ]
        };

greatest values for each key:
22: 18.25
21: 16.25