And that's not even considering leaks or bugs, that's just considering the current design.

To be fair, I believe that they didn't mean to say memory leak is feature. We all know that, to use big amount of memory is different from momery leak.

I guess what they meant was that the current thread design and implementation in Perl took lots of memory.

I think what they tried to do is to make it work first, in a easier way, and try to cut down memory usage at a later point. I agree it is not ideal, and not a good design, but I believe that all of us, at some point, have experienced the situation where you have to deliver your project phase by phase.

Really, take a look at 5.10 doc on thread, they are fixing the memory leaks, and they also did something to cut down the meory usage for thread.