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?
"called anywhere from every minute to many times per second. As a consequence, keeping it's memory and startup time low are important."
This doesn't sound like a script to me at all.
This sounds like a server handling requests that arrive from every minute to several times a second.
If it's a server, handling requests, start-up time doesn't mean as much as responsiveness.  In which case, Python might work out well, and still keep performance up.
Rather than restarting, you're just processing another request.  You get to keep as much state as you need to optimize performance.
Scripting language choice for initial performance