for my $key (sort keys %$data) {
    # $key should be 1, 2, then 3 here...
    for my $subject (sort keys {$data->{$key}) {
        # $subject should be subject1, then subject 2 here
        if ($subject eq "subject1") {
            print "$data->{$key}{$subject}\n";
        }
        elsif ($subject eq "subject2" {
            # subject2 has arrayref of one item,
            # an href, with two keys:
            print "$data->{$key}{$subject}[0]{subject3}\n";
            print "$data->{$key}{$subject}[0]{subject4}\n";
        }
    }
}