You are forgetting not mentioning that there are actualy two pieces of data there that need untainting: one is the domain parameter obtained from the CGI, but the other is the PATH of your program. If you use backticks like that, and don't set up your PATH explicitly, perl -T will complain.

That appears not to make sense in a CGI environment, but it makes a lot of sense when you're writing setuid root scripts that can be run from the command line.