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++;
}


##</code><code>##

for my $i ( 0 .. length( $dna ) - 1 ) {


##</code><code>##

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