No need to parse it manually

my $param = do {
   local $ENV{'REQUEST_METHOD'} = 'GET';
   local $ENV{'CONTENT_LENGTH'};
   local @QUERY_PARAM;

   my $cgi = CGI->new();
   
   $cgi->param(...)
};

my $cgi;
if ($param) {
   $cgi = CGI->new(\&hook);
} else {
   $cgi = CGI->new();
}
[download]

Untested, even less so with mod_perl.

Update: Added local @QUERY_PARAM;.