What's happening is that the $param method is returning a null list in this case, not undef. This null list results in nothing being passed to bind_param from $q->param('id'). The reason that it's doing this is that it's being called in list context being in an argument list, and thus CGI.pm thinks it needs to return a (possibly empty) list. You should be able to fix this by explictly specifying scalar context with something like scalar $q->param('id').