my $next = hash_walker(\%data);
while (my @row = $next->()) {
    print join(',', @row), "\n";
}
sub hash_walker {
    my ($href) = @_;
    die "Not a hash ref" if ref($href) ne 'HASH';
    my (@ref, @key) = ($href, ());
    while (ref($href) eq 'HASH') {
        my ($key, $val) = each %$href;
        push @key, $key;
        push @ref, $val;
        $href = $val;
    }
    my ($val, $done) = (pop @ref, undef);
    return sub {
        return () if $done;
        my @row = (@key, $val);
        my ($k, $v) = each %{$ref[-1]};
        if (defined $k) {
            ($key[-1], $val) = ($k, $v);
            return @row;
        }
        my $idx = $#ref;
        while ($idx--) {
            my ($k, $v) = each %{$ref[$idx]};
            next if ! defined $k; 
            $key[$idx] = $k;
            $ref[$idx + 1] = $v;
            for ($idx + 1 .. $#ref) {
                my ($k, $v) = each %{$ref[$_]};
                $key[$_] = $k;
                $ref[$_ + 1] = $v;
            }
            $val = pop @ref;
            return @row;
        }
        $done = 1;
        return @row;
    };
}
[download]

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";
    }
}
[download]