my $word = 'layer';
my $new_word = join '', map { "[$_]" } split '', $word;

# Or, if I want to be more explicit about the process:
# my @raw_chars = split '', $word;
# my @cooked_chars = map { "[$_]" } @raw_chars;
# my $new_word = join '', @cooked_chars;

##</code><code>##

$ perl -E '$word = "layer123 $-.% foo"; $new_word = join "", map { $_ =~ /[a-zA-Z]/ ? "[$_]" : $_ } split "", $word; say $new_word;'
[l][a][y][e][r]123 0.% [f][o][o]