I have a small lightweight application that is used as part of a larger solution. Currently it is written in C but I am looking to rewrite it using a cross-platform scripting language. The solution needs to run on Windows, Linux, Solaris, AIX and HP-UX.
The existing C application works fine but I want to have a single script I can maintain for all platforms. At the same time, I do not want to lose a lot of performance but am willing to lose some.
Startup cost of the script is very important. This script can be called anywhere from every minute to many times per second. As a consequence, keeping it's memory and startup time low are important.
So basically I'm looking for the best scripting languages that is:

Cross platform.
Capable of XML parsing and HTTP Posts.
Low memory and low startup time.

Possible choices include but are not limited to: bash/ksh + curl, Perl, Python and Ruby. What would you recommend for this type of a scenario?
I concur with Lua: it is super-portable, it has XML libraries, either native or by binding C libraries like Expat, it has a good socket library (LuaSocket) plus, for complex stuff, some cURL bindings, and is well known for being very lightweight (often embedded in low memory devices), very fast (one of the fastest scripting languages), and powerful. And very easy to code!
It is coded in pure Ansi C, and lot of people claim it has one of the best C biding API (calling C routines from Lua, calling Lua code from C...).
Scripting language choice for initial performance