As I said, I do not really know the details. This is not my main work, but an application for which I only work occasionally. It was previously working on 5.8, if I remember correctly (or perhaps 5.10, but I think it was 5.8), and they decided to upgrade because they wanted to use threads in a new framework developped internaly, and, apparently, it does make a difference. I do not know the details personally, but I would tend to trust the person who proposed that upgrade decision, because I know it is someone who usually knows fairly well what he is doing.

But the point is that the improvement was compared to Perl 5.8, not a more recent version.