use strict;
use warnings;
use Data::Dumper;

my $s1 = "STCATTNNNSATCGCT";
my $s2 = "ATCGTCGSNNNNATCG";
$s1 =~ tr/N/n/;
my $merge = $s1 ^ $s2;
my $count = $merge =~ tr/\0//;

print $count;

##</code><code>##

4