$N =~ tr|[0-9]||cd;

NB: The first field of the  tr/// operator (see Quote-Like Operators in perlop) is a search list, and it does not "know" about  [...] character classes (the search list is itself a kind of character class), so the  [ ] characters are among the "digit" characters which are not deleted.

c:\@Work\Perl\monks>perl -wMstrict -le
"my $n = 'abc[]1p2q3][xyz';
 $n =~ tr|[0-9]||cd;
 print qq{'$n'};
"
'[]123]['
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Don't know about the rest of the question.