How about just tr'ing the initial Base64 ID like so:

$id =~ tr|+/=|___|;
# or
$id =~ tr|+/=|000|;
[download]

You would lose just a few bits of randomness (acceptable in this application), but would be left with a shorter ID that's an easy match with a /\w/.

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print