#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=766743
use warnings;

my %thexor; # check for N mismatches different from non-N mismatches
for my $x ( qw( A T G C N ) )
  {
  for my $y ( qw( A T G C N ) )
    {
    $x lt $y and $thexor{$x ^ $y} .= "$x$y ";
    }
  }
use Data::Dump 'dd', 'pp'; dd \%thexor; # yes they are

# mismatch "\2\4\6\23\25\27"
#    match "\0\t\r\17\32"

local $/ = '';
while( <DATA> )
  {
  my ($x, $y) = split;
  my $bad = ($x ^ $y) =~ tr/\2\4\6\23\25\27//; # therefore this counts mismatches
  print "$x ^ $y => ", pp($x ^ $y), $bad ? ' FAIL' : ' ok', "\n";
  }

__DATA__
ATGNCNC
ATGACNN

ATGNCNC
TTGNNNC