Heres a short example that does that:

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
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And yet another example!:

use strict;
use warnings;

my $string = "this is a test";
my $modified_string = $string =~ s/([a-zA-Z])/\[$1\]/gr;
print $modified_string;
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EDIT: changed pattern match to include only characters a-z in first example.