The following should work, no matter what encoding was used by the source string.

MIME::WordDecoder->default(MIME::WordDecoder->supported("UTF-8"));

$subj = decode('UTF-8', unmime($subj));
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Update: ug! Don't use MIME::WordDecoder. After looking at its guts, I'd recommend the snippet I presented earlier (packaged as a reusable function below). I think this toolkit predates the addition of UNICODE support to Perl. That would explain why the weird and convoluted interface.

use MIME::Words qw( decode_mimewords );
use Encode      qw( decode );

sub mime_decode {
   my $decoded = '';
   foreach (decode_mimewords($_[0])) {
      my ($data, $charset) = @$_;
      if (defined($charset)) {
         $decoded .= decode($charset, $data);
      } else {
         $decoded .= $data;
      }
   }
   return $decoded;
}

$subj = mime_decode($subj);
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Untested