output_hash(\%data);

sub output_hash {
    my ($href) = @_;
    die "Not a hashref" if ref($href) ne 'HASH';
    my (@work, @row) = ($href, ());
    while (@work) {
        my $item = pop @work;
        my ($k, $v) = each %$item;
        next if ! defined $k;
        if (ref($v) eq 'HASH') {
            push @work, $item, $v;
            $row[$#work - 1] = $k;
            next;
        }
        push @work, $item;
        print join(',', @row, $k, $v), "\n";
    }
}