MMORPG servers.

I'd say Perl is your perfect choice. I can't attest to the merits of Python but lets look at your requirements list shall we?

3d coordinate system

Math::Big
Games::3D - looks like an interesting starting point
Use big floats to define your object base 3D. Use nested relative rather than global absolute system if you want the universe to grow without bounds.

physics

Physics::Particles
Physics::Springs

collision detection

Math::Matrix
Math::Vector

network connections

Perls net capabilities are vast, start by looking at these.
Net::Server
IO::Socket
Net::Socket

authentication

Authen::* - all kinds of authentication

NPC AI

Games::AIBot
AI:Fuzzy
AI:Categoriser

battles

Games::Object
Games::Maze

simple-but-applicable benchmarks?

I've written a couple of games using UScript (Tim Sweenys Unreal scripting language) which is fairly clunky interpreted Java. I would put it alongside Perl in terms of practical all round speed, so I don't see any reasons Perls performance should bother you. The only hardcore code needed is where you have collision detection for hundreds of objects going on at once, and/or too many traces. If you want real time action physics you shouldn't be using a fat server model anyhow, you need replicated object physics and you will need C for speed here. As I understand it MMORPGs span many servers, so you should build 'load balacing' right into the framework of your universe, that is each server handles parts of the universe that are not only local and relevant to it, but are within its capabilities to compute. For a complex and diverse problem like this, I really can't think of a better language than Perl.