The previous answers were pretty good but in case you are still confused here you go:

my @array = qw(A R N D);
# is equivalent to
my @array = ();
$array[0] = 'A';
$array[1] = 'R';
$array[2] = 'N';
$array[3] = 'D';
[download]

So it should be obvious to you how you can overwrite any part of the array...simply reference it by its index:

$array[0] = 'B';
[download]