The reason it won't work is because by default tr, transliterates, or changes, character for character between the search list to the replacement list. However if you stick a 'd' on the end it will delete everything in the search list that doesn't have a corresponding replacement e.g

$var =~ tr/\\//d;    # \ deleted, nothing to replace
$var =~ tr/\\/X/d;   # changed \ into X
[download]

As someone is bound to say RTFM (man perlop).
HTH

broquaint