Why would you want this? It takes care of the parsing, letting you write very simple CGIs in your favourite scripting language, e.g. sh.
I gave a talk about shell scripting on the web (using urldecode) at Techknowfile 2012, and more information about its use are available in the slides.
urldecode is available in binary or source form at ftp://ftp.cs.toronto.edu/pub/jdd/urldecode. It should run on pretty much any standard system.
Comments, bug reports, fixes, enhancements, etc. are welcome.