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 agree with others in that you should probably try to make this a more portable C app instead of porting it over to something else since any scripting language is going to introduce significant overhead from a startup perspective, have a much larger memory footprint, and will probably be much slower.
In my experience, Python is the most efficient of the three, followed by Perl and then Ruby with the difference between Perl and Ruby being particularly large in certain areas.  If you really want to try porting this to a scripting language, I would put together a prototype in the language you are most comfortable with and see if it comes close to your requirements.  If you don't have a preference, start with Python as it is easy to learn and use and if it is too slow with Python, Perl and Ruby probably won't be able to do any better.
Scripting language choice for initial performance