Select Code to Download

my $s1 = "STCATTNNNSATCGCT"; 
my $s2 = "ATCGTCGSNNNNATCG";
...

# There are 4 matching positions - marked with (#) -
# and notice that matching N is not counted (0)

my $match = $s1 ^ $s2;
my $match_count = $match =~ tr/\00//;
...

# This gives 5, because N is counted.
# How can I make it to return 4 instead?

get_match_count($s1,$s2);
...
    print "$num_match\n";
    return $num_match;
}