I think this is is a contributor to why Perl 6 will never amount to anything. That boat has sailed. Python 3 broke backwards compatibility and to my knowledge everybody is still using Python 2. Python has a lot of energy right now, so that might save it, or it might go the way of Perl when the next cool language comes out and everybody jumps ship.
If you count big chunks of legacy python 2 code a "minor exception", I agree.
Anyway, the big difference between python 2 -> 3 transition and the Perl 5 and Perl 6 coexistence is that nobody within the Perl community tried to end-of-live Perl 5.
Yes, that means less incentives for migrating to Perl 6, but it also reduces the risk for Perl 5 users.
While this is true, it's possible to write a script that will run on both Python2 and Python3, so long as you have that goal in mind. The breaking of backwards compatibility is the breaking of several APIs and assumptions.
> to my knowledge everybody is still using Python 2
This is mostly because the release of Python 3 didn't see everyone stop everything and immediately start coding in Python 3. Conversion has to bubble up from the major libraries (e.g. Django, Flask, SciPy, etc) first, before people are able to convert.
Also, switching the default Python on Linux distros takes time because you have to go through all of the packages in the package manager looking for Python scripts and make sure that anything pointed at /usr/bin/python will work with Python 3[1].
Another point is that a number of libraries delayed conversion, and didn't consider Python3 to be "stable" until around Python 3.3. Some of other libraries (cough Flask cough) delayed support for a long time because the maintainer just didn't like Python 3.
[1] I know that these aren't always working well though. A while back I found out that Comix (installed via Ubuntu) had "/usr/bin/env python" in the shebang line instead. Launching it from a Virtualenv that doesn't have PyGTK doesn't work so well... :P
The big problem with Perl 6 has been that the implementation has trailed the ambitions of the design. But that's changing due to pieces like Inline::Perl5 falling in to place. (See my comments nearby for more details.)
In the unlikely event there's a module that doesn't work, then yes, you have to learn and wield a new language that isn't backwards compatible. But it is Perlish, and it is designed to coexist beautifully with Perl 5.
Perl 6 is for new projects. Perl 5 ecosystem continues evolve untouched. Unlike the Python 2 scenario, where 2.x won't be supported and 3.x series and its library ecosystem isn't ready anymore.
