@hypolite I found this article... help.dreamhost.com/hc/en-us/ar… Apparently FastCGI is more problematic than I realized:

You cannot get the query string in FastCGI by using the @ARGV array. You must use $ENV{'QUERY_STRING'} instead.
and...
If an error occurs in your FastCGI script, it may not be returned immediately. If the script loads a web page, the page will appear to hang. When the script times out, it's possible that the error message may not be logged and you'll only see a 500 error.