You could use arrays to store your results. Note where you say print "$last only = $first\n" you haven't tested if last eq first, only if last eq second.

my (@both,@last_second,@last_first);
foreach $line (@sorted_data) {
    chomp $line;
    $line =~ /(.*?)\,(.*?)\,(.*)/;
    $first = $1;
    $second = $2;
    $last = $3;
    if ($last eq $second and $second eq $first) {
        push(@both,"$last = $second and $second = $first");
    }
    elsif ($last eq $second) {
        push(@last_second,"$last = $second");
    }
    elsif ($last eq $first) {
        push(@last_first,"$last = $first");
    }
}
print "both\n", join("\n",@both), "\n";
print "last_second\n", join("\n",@last_second), "\n";
print "last_first\n", join("\n",@last_first), "\n";
[download]