Hi I have written a simple program to print a string with one character of the string changed to either A,C,G or T. This is a simple bioinformatics program to generate a SNP. I see the output with as well as without the print statement of the subroutine for some unknown reason. 

Actual possible output (out of many expected) should be:
The DNA sequence is:
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
 mutation at base position 5
AAAATAAAAAAAAAAAAAAAAAAAAAAAAA

But sometimes the following is obtained:
The DNA sequence is:
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

[download]
(Here the sub routince call is not considered...why?)

Can anybody help me with the reason for this. Thanks in advance, minnie

Program code is as follows: 
#!/usr/bin/perl -w

print "The DNA sequence is : \n" ;
my $seq = 'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA';

print "$seq \n";
mutate ($seq);
print "\n";
exit;

sub mutate{
my($dna) = @_;
my $position = randposition($dna);


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++;
}


}

sub randposition{
my($sequence) = @_;
return int rand length($sequence);
}

sub randombase{

my(@nucleo) =('A','T','C','G');
return $nucleo[rand(@nucleo)];
}

[download]

In reply to Help regarding the outout of the program by minnie

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