The install needs to be interactive for reasons other than the installation location. At perl Build.PL time lots of questions get asked, related to things such as which database should be used during testing, which user to connect as, etc. These things cannot be guessed reliably and so the user must be prompted for them.

As the user is being prompted for these details it is only sensible to ask for the installation path as well through a prompt, rather than getting the user to use another method to set that one value.

If the installation path was the only thing that needed to be determined and it could default to the system standard then I would agree with you, however in _this_ situation I feel that prompting in a consistent manner is the best approach.