my $str  = 'AGACGAGTA';

my $mask = "\xFF" x length($str);
substr($mask, $_, 1, "\x00") for 2..6;

my $res = $str & $mask;
$res =~ s/\x00/x/g;

print("$res\n");

##</code><code>##

my $str = 'AGACGAGTA';

my $mask = "\xFF" x length($str);
substr($mask, $_, 1, "\x00") for 2..6;

my $x = 'x' x length($str);
my $res = ($str & $mask) | ($x & ~$mask);

print("$res\n");

##</code><code>##

my $str = 'AGACGAGTA';

my $res = $str;
substr($res, $_, 1, "x") for 2..6;

print("$res\n");