So you want a simple explanation of why it's complex, eh?
If you don't mind complex explanations of the complexity, those can be found in the apocalypses and synopses. And some of my talks have been about why the parser is complex. I generally run out of time in such talks... :-)
For a more academic view of the complexity of what we're trying to do, here are some other people trying to do (some of) the same things with a different language, Fortress: "Growing a Syntax".
It's easy to argue that we could have done this or that differently, and spend even more energy arguing whether those things were actually possible at the time or would have simply resulted in other dystopias. But the fact is that, within the constraints of health, sanity, and politics, we took the best shot at it we knew how, and our decisions, right or wrong, have put us where we are today, with a great deal of improvement in both Perl 5 and Perl 6 over the last ten years, in absolute terms.
It's the relative worth of Perl with respect to other languages, and the perceptions and misperceptions of that worth, and the perceptions and misperceptions of other people's perceptions and misperceptions, that drives people's fears. And "Fear is the mind killer."
So I simply say, "Don't do that." Regardless of our mistakes and our perceived mistakes, the Perl 5 and Perl 6 communities will both continue to prosper, because we have iron-willed people who are not going to give up. You can't change the past. You can't even change the future, in the sense that you can only change the present one moment at a time, stubbornly, until the future unwinds itself into the stories of our lives. My father always used to say "It's not what you do, it's what you do next." So let's all look toward the future, and keep on doing the next thing, and the next, and the next.
In reply to Re^4: A wholly inadequate reply to an Anonymous Monk