for ($i =0;$i<length($dna);$i++){

    if ($i == $position){

print " mutation at  base position ". ($position+1) . "\n";
    do {
    $nucleotide = randombase();

     } until ($nucleotide ne substr($dna,$position,1));
    substr($dna, $position, 1, $nucleotide);

    print "$dna\n";
    }
$i++;
}