Hmm, can you run it from the command line? Typically when I get Internal Server Errors, I try running my CGI script from the command line and I get a nice descriptive error very quickly. You can also look at the Apache error log as others have mentioned.

Also, when you say "inherited", do you mean that you are maintaining it in the exact same directory as it came from, or have you moved it around the filesystem? That could also give you problems if you know it otherwise worked earlier.