When in doubt, trust the actual Perl documentation over any other website. Type "perldoc perlop" at your command line.

The issue with the unofficial documentation you found is that tr does not take special regular expression escapes like "\d" or bracketed classes like "[abc]" in its arguments.

Note that "tr" does not do regular expression character classes such as "\d" or "[:lower:]".

$string =~ tr/0123456789/ /c;
[download]

It does support ranges, but use no brackets. Shortcut the above with the special hyphen:

$string =~ tr/0-9/ /c;
[download]