I just grabbed WebInject yesterday, as I was looking to add some extended monitoring to Nagios.
My original plan was to install webinject in /usr/local/bin, and have the scripts located under /usr/local/nagios/libexec/scripts. This brought up an interesting bug, which I see was mentioned here and here. It looks like work is being done on the second one.
The other issue is related to temporary files. With the above setup in mind, the nagios user wouldn't have write access to anything but the nagios/var directory, however, it seems that WebInject wants to take a temporary file, and put it inside the directory where the testcases are located. This should be easily changed out to using something simple like mktemp (or Perl equivalent).
Originally reported by: Jon Angliss