From perlop:

tr/SEARCHLIST/REPLACEMENTLIST/cds
y/SEARCHLIST/REPLACEMENTLIST/cds
Transliterates all occurrences of the characters found in the search list with the corresponding character in the replacement list. It returns the number of characters replaced or deleted. If no string is specified via the =~ or !~ operator, the $_ string is transliterated. (The string specified with =~ must be a scalar variable, an array element, a hash element, or an assignment to one of those, i.e., an lvalue.)

From perlrequick:

Search and replace
Search and replace is performed using "s/regex/replacement/modifiers". The "replacement" is a Perl double quoted string that replaces in the string whatever is matched with the "regex". The operator "=~" is also used here to associate a string with "s///". If matching against $_, the "$_ =~" can be dropped. If there is a match, "s///" returns the number of substitutions made, otherwise it returns false.

Ciao, Valerio