A CGI script needs to output at least a Content-Type header, followed by an empty line (which indicates the end of the headers section), before the actual content begins... As you've already loaded the CGI module, this is most easily done with print header; anywhere before your print $QueryString;.

BTW, I hope only trusted users are having access to that URI... Otherwise, directly interpolating the QUERY_STRING into an SQL query without any further checks is a very bad idea, security-wise... (better use placeholders)