tab@music3:~/2018-0717$ cat pm.pl 
#!/usr/bin/perl

use strict;
use warnings;

{
    my $word = '$-.%';
    my $new_word = join ( '', map { "[$_]" } split /[a-zA-Z]/, $word );

    print "$word -> $new_word\n";
}
tab@music3:~/2018-0717$ perl pm.pl 
$-.% -> [$-.%]
tab@music3:~/2018-0717$