# Solution 2:
my $s_mask = unpack ("b*",$mask);
my $pos=0;
print "Res:  ";
while ($pos < length($seq)){
  no warnings;  ## $s_mask could be shorter than $seq
  print ! substr($s_mask,$pos,1) ? substr($seq,$pos,1) : "x";
  $pos++;
}
print "\n";