I think a split-based solution (which I personally would not use) would have to look something like this to get what seems (I think) to be the required output:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $word = '$-.%aBc&/';
 my $new_word =
   join '',
   map { /[[:alpha:]]/ ? qq{[$_]} : $_ }
   split '',
   $word
   ;
 print qq{'$word' -> '$new_word'};
"
'$-.%aBc&/' -> '$-.%[a][B][c]&/'
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Since this depends on two regexes, I think you might as well use a  s/// approach and be done with it.