Okay, I was thinking @QUERY_PARAMS was getting reset from the empty param() list somewhere in there, but that doesn't seem to be the case.

I would use the $CGI::PERLEX setting, since all it does it flag _reset_globals() to be called in the constructor and is pretty much a "set it and forget it" thing. Of course, 'undef @CGI::QUERY_PARAM' may be the only method guaranteed to work with future versions.