use strict;
use warnings;

my $modified_string;

while ( read DATA, my $buf, 1 ) {
    $modified_string .= "[$buf]" unless $buf !~ /[a-zA-Z]/;
}

print $modified_string;

 __DATA__
this is a test


##</code><code>##

use strict;
use warnings;

my $string = "this is a test";
my $modified_string = $string =~ s/([a-zA-Z])/\[$1\]/gr;
print $modified_string;