I use this technique to ferret out these errors:

    $SIG{__WARN__} =
      sub {
           warn @_;
           $i=0;
           while(($pack, $file, $line, $subname, $hashargs, $wantary)
                 = caller($i++)){
               print STDERR
                   "  WARN pack='$pack' file='$file' line='$line' ",
                   "subname='$subname'\n";
          }
    }
[download]

I cant claim authorship of the code, I found it by googling the newsgroups...