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?
When written properly, C should be platform independant and would only need a recompile for those different platforms. You might have to jump through some #ifdef hoops for the headers (not all systems use the same headers), but most normal (non-win32 API) calls are very portable.
For web access (which I presume you need as you mention bash+curl), you could take a look at libcurl, it's available for all the platforms you mentioned, and shouldn't be that hard to work with.
With execution time and memory cost in mind, I doubt you could go any faster than properly written C with any scripting language as you would lose at least some time on interpreting the script...
Scripting language choice for initial performance