From the perldoc:

tr/SEARCHLIST/REPLACEMENTLIST/cdsr
y/SEARCHLIST/REPLACEMENTLIST/cdsr

...

Options:

    c Complement the SEARCHLIST.
    d Delete found but unreplaced characters.
    s Squash duplicate replaced characters.
    r Return the modified string and leave the original string
    untouched.

If the /c modifier is specified, the SEARCHLIST character set is compl
+emented.
If the /d modifier is specified, any characters specified by SEARCHLIS
+T not
found in REPLACEMENTLIST are deleted. (Note that this is slightly more
+ flexible
than the behavior of some tr programs, which delete anything they find
+ in the
SEARCHLIST, period.) If the /s modifier is specified, sequences of cha
+racters
that were transliterated to the same character are squashed down to a 
+single
instance of the character.

If the /d modifier is used, the REPLACEMENTLIST is always interpreted 
+exactly as
specified.
[download]

NOTE! the /r option is relatively new and may not be present in your version of Perl. Check your local docs to be sure.