my @array = split //, $string;
my @newarray;
my $c = shift @array;
while ( @array ) {
    my $d = shift @array;
    push @newarray, $c;
    if ( $c eq $d ) {
       push @newarray, 'x';
    }
    $c = $d;
}
push @newarray, $c;
$string = join '', @newarray;  #update