That's simple to try .. I suspect you didn't try it.

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

use strict;
use warnings;

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

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

Perl doesn't care what's in the string -- more specifically, split doesn't have problems with any characters that may have special meaning in a regex.

$-.% -> [$][-][.][%]

Note the OP said (emphasis mine):

I need to find any letter [a-zA-Z] and replace it

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.

Alex / talexb / Toronto

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