for my $let (keys %hash) {
    for my $num (keys %{$hash{$let}}) {
        for my $name (keys %{$hash{$let}{$num}}) {
            print join(',', $let, $num, $name, $hash{$let}{$num}{$name}), "\n";
        }
    }
}

##</code><code>##

output_hash(\%data, []);

sub output_hash {
    my ($href, $item) = @_;
    die "Not a hashref" if ref($href) ne 'HASH';
    my ($key, $val) = each %$href;
    if (ref($val) ne 'HASH') {
        for my $key (sort keys %$href) {
            print join(',', @$item, $key, $href->{$key}), "\n";
        }
        return;
    }
    for my $key (sort keys %$href) {
        push @$item, $key;
        output_hash($href->{$key}, $item);
        pop @$item;
    }
}