Ugh .. I hate mis-reading the specs.

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$
[download]

In addition, I should have used // in my original call to split, as it takes a regex and not a string. Thanks for the reminder.

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

Sorry, I think that's also not right:

$ perl -wMstrict -MData::Dump -e \
    'dd join "", map { "[$_]" } split /[a-zA-Z]/, q($-.%abc&/)';
"[\$-.%][][][&/]"
[download]

Whereas, by my understanding of the question (that does still need to be clarified), the output should be '$-.%[a][b][c]&/'.

Good point -- I was only answering the follow-up, not the reply. And now I've spent an entertaining hour tinkering with Perl regular expressions, time which is never wasted. :)

Here's my improved code, which shows that I still have some distance to go in writing regular expressions:

tab@music3:~/2018-0717$ cat !$
cat pm2.pl
#!/usr/bin/perl

use strict;
use warnings;

{
  my $word = q($-.%abc&/);

  my @group1 = ( $word =~
    m/(?:            # Non-capturing group of captured
      ([^a-zA-Z]+)   # .. non-matching
      ([a-zA-Z]+)    # .. following by matching
     )+              # .. for as many groups as possible
     ([^a-zA-Z]+)    # followed by matching.
    /x );

    my @group2 =
      map { $_ =~ /[a-zA-Z]/ ? split( //, $_ ) : $_ } @group1;

    my $new_word = join ( '', map { "[$_]" } @group2 );

    print "$word -> $new_word\n";
}
tab@music3:~/2018-0717$ perl pm2.pl
$-.%abc&/ -> [$-.%][a][b][c][&/]
tab@music3:~/2018-0717$
[download]

Alex / talexb / Toronto

Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.