Perl6 is incompatible with Perl5, so they're also going through a similarly painful transition.
Everyone waxes lyrical about how Python 2.x was "good enough and why would you change it", but there were several things in Python 2.x that were objectively awful (unicode was broken, iterator variables can leak to the outer scope, division of integers producing integers, xrange, raw_input, mixed indentation "working", etc). And while no single issue alone would justify Python 3.x, the combination of all of these issues as well as some other semantic problems justified the need for Python 3.
Of course, that being said, Python 3 also had some issues that it took several versions to iron out.