Perl does NOT have a lightweight threading model. Period. Literally every time you spawn a thread, Perl copies all of its data structures to avoid sharing stuff that is not threadsafe. If your Perl developers try to tell you otherwise, they are incompetent. The standard way to build a high performance website in Perl is to use prefork and then put a reverse proxy in front of them in httpd accelerator mode. See http://perl.apache.org/docs/1.0/guide/strategy.html for an old, but still accurate, description of httpd accelerator mode if you don't know what it is.
Please note that Windows does not have good support for forking. Therefore I have never heard of a competent team of Perl developers choosing to try to deploy a high volume website on Windows.
To the best of my knowledge both Ruby and Python do support lightweight threading, unlike Perl, but both have a global interpreter lock that synchronizes very frequently, and severely limits scalability. I don't know what strategies are common in those languages, but personally I suspect that the right strategy on Unix with those languages is, like Perl, to use pre-fork with a reverse proxy in httpd accelerator mode.
However both Python and Ruby have implementations on other platforms, such as the JVM. I believe (verify this before relying on it) that those implementations support lightweight threads without the drawbacks of the C implementation. Therefore if you want to use lightweight threading with that class of scripting language, use one of those implementations.