It is whatever it is configured to be on that particular web server. A web server could be configured to run .pl files with the php module and .aspx files with perl, although that would be silly. There are no scripts involved with most web servers, instead you’d have to look in your apache configuration files (or equivalent, if using different server software). If you have permission to edit the server config file, then you could make files ending in .do run as php, if that’s what you’re after.
