johngg@shiraz:~/perl/Monks > perl -Mstrict -Mwarnings -E '
my @arr1  = qw{ A T C G T C G A G C G };
my @arr2  = qw{ A C G T C C T G T C G };
my $count =
   ( ( join q{}, @arr1 ) ^ ( join q{}, @arr2 ) ) =~ tr{\0}{};
say $count;'
4