Use of uninitialized value in substr

[elavazhagan perl]

Hai



While executing the mutate.pl pgrogram,the diagnostic msg obtained as

"Use of uninitialized value in substr at mutate.pl line 31, <STDIN> line 1."

I almost defined all the variables. Would you kindly provide some remedy to
this……

This is the mutate  programm...............................
#!/usr/bin/perl
#mutate.pl
use strict;
use warnings;
print " Enter the DNA Sequences";
my $DNA;
my @nucleotide;

chomp($DNA = <STDIN>);
my $i;
my $mutant;
srand(time|$$);
$mutant=mutate($DNA);
print "Mutate DNA\n\n";
print " Here is the original DNA\n\n";
print "$DNA\n";
print "Here is the mutant DNA \n\n";
print "$mutant\n\n";
print "Here are 10 successive mutations\n\n";
for ( $i=0;$i<=10;++$i)
 {
$mutant=mutate($mutant);

print ("$mutant\n\n");
  }
exit;
sub mutate {
my ($DNA) [EMAIL PROTECTED];
my ($position)=randomposition($DNA);
my ($newbase)=randomnucleotide();
substr($DNA,$position,1,$newbase);
return $DNA;
          }
sub randomposition {
my($string)[EMAIL PROTECTED];
return  int rand length $string;
                  }
sub randomelement {
my(@array)= @_;
return $array[rand @array];
                 }
sub randomnucleotide {
my ($nucleotides) = ('A','G','C','T');
return randomelement(@nucleotide);
                    }