Excellent example; but consider use of a character class (see perlre rather than specifying all 52 lc & uc letters from a to z and the 10 digits from 0 to 9...

[a-zA-Z0-9]

C:\>perl -E "my $s='abcdeEFGHI20130619ABC';$s =~ tr/[b-dB-D2-5]/*/;say
+ $s;"
a***eEFGHI*01*0619A**
[download]

update: restating for specificity: 52 uc and lc; 10 digits

update2: choroba points out that (s)quare brackets are not special in tr///; ie, that my suggestion of a char class is wrong -- something illustrated by this:

C:\>perl -E "my $s='abcdeEFGHI20130619ABC';$s =~ tr/b-dB-D2-5/*/;say $
+s;"
a***eEFGHI*01*0619A**
[download]

Mea Culpa.