No offense, but like the other question, you kinda need to go and read up on mod_perl programming before asking a bazillion questions.

No, this is not a taint issue. It's exactly what the error says: $ENV{QUERY_STRING} is not defined. Why is it not defined? Because you're running under mod_perl, not mod_cgi

Basically, $ENV{QUERY_STRING} is only available when you're running under mod_cgi, which populates %ENV with some CGI-specific information.

mod_perl doesn't do that. All the information is in the Apache object ($r).

If you insist on using Apache::Registry, you should stop sending headers with print and trying to get the query string directly -- just use CGI.pm, which handles these things correctly for you