Good prototyping software

All the code for my recent paper was written in MATLAB. it was convenient, especially since a lot of the prior work was in MATLAB too. I actually know almost no MATLAB, preferring to do my rapid protoptyping in C++ (yes, I'm crazy, I know).

Which brings me to the question that I know my lurking software hackers might have an answer to. If I have to invest in learning a new language for doing the empirical side of my work, what should it be ? Here are some desiderata:

• Extensive package base: I don't want to reinvent wheels if I can avoid it. In this respect, the C++ STL is great, as is Boost, and Python has PADS, as well as many nifty packages. MATLAB is of course excellent.
• Portability: I'm sure there's an exotic language out there that does EXACTLY what I need in 0.33 lines of code. But if I write code, I want to be able to put it out there for people to use, and I'd like to use it across multiple platforms. So Lua, not so great (at least in the research community) (sorry Otfried)
• Good I/O modules: if I write code, I'll often want to send output to a graph, or plot some pictures etc. Some systems (MATLAB) are excellent for graphing data.
  
• Performance: I don't want to sacrifice performance too much for ease of coding. I've always been afraid of things like Java for this reason. Of course, I'm told I'm dead wrong about this.
  

I deliberately haven't listed 'learning curve' as an option. If the language merits it, I'm willing to invest more time in switching, but obviously the benefits have to pay for the time spent. In terms of background, I'm most familiar with C++, and am a nodding acquaintance of python, and perl. I occasionally nod at MATLAB in the street when I see it, but usually cross the road to the other side if I see Java approaching. I used to be BFF with OpenGL, but then we broke up over finances (specifically the cost of GPUs).

Thoughts ?