use strict;
use warnings;

my ( %individuals_by_first_name, %individuals_by_middle_name);

foreach my $line (<DATA>) {
    chomp($line);
    my ($first, $middle, $last, $age, $sex, $location) = split(/,/, $line);
    my $record = [ $first, $middle, $last, $age, $sex, $location ];
    push(@{$individuals_by_first_name{$first}}, $record);
    push(@{$individuals_by_middle_name{$middle}}, $record);
}

foreach my $first (sort keys %individuals_by_first_name) {
    foreach my $individual (@{$individuals_by_first_name{$first}}) {
        my ($first, $middle, $last, $age, $sex, $location) = @$individual;

        print "$first: $middle, $last, $age, $sex, $location\n";
    }
}

__DATA__
first1,middle1,last1,27,M,here
first2,middle2,last2,56,M,there
first3,middle3,last3,30,F,everywhere
first1,middle4,last4,7,F,home
first4,middle2,last3,22,M,away

##</code><code>##

first1: middle1, last1, 27, M, here
first1: middle4, last4, 7, F, home
first2: middle2, last2, 56, M, there
first3: middle3, last3, 30, F, everywhere
first4: middle2, last3, 22, M, away

##</code><code>##

use strict;
use warnings;

my %individuals;

foreach my $line (<DATA>) {
    chomp($line);
    my ($first, $middle, $last, $age, $sex, $location) = split(/,/, $line);
    my $record = [ $first, $middle, $last, $age, $sex, $location ];
    push(@{$individuals{first}{$first}}, $record);
    push(@{$individuals{middle}{$middle}}, $record);
}

foreach my $first (sort keys %{$individuals{first}} ) {
    foreach my $individual (@{$individuals{first}{$first}}) {
        my ($first, $middle, $last, $age, $sex, $location) = @$individual;

        print "$first: $middle, $last, $age, $sex, $location\n";
    }
}

__DATA__
first1,middle1,last1,27,M,here
first2,middle2,last2,56,M,there
first3,middle3,last3,30,F,everywhere
first1,middle4,last4,7,F,home
first4,middle2,last3,22,M,away