Check out perldoc perlop for the options to tr. As it appears our site docs are still a bit old, here is an excerpt from a Perl5.6.0 dist:

c   Complement the SEARCHLIST.
d   Delete found but unreplaced characters.
s   Squash duplicate replaced characters.
U   Translate to/from UTF-8.
C   Translate to/from 8-bit char (octet). 

jeffa