First of all, the quick answer: declare $count = -1; somewhere earlier.

Next the long answer. Your code is not good for a variety of reasons, most of which you learn from practice and experience, which if you hang out here you will gain quickly. Try this:

use strict; # always.
my $drive;
my $count = -1;
open( LIST, "list -l|" ) or die $!;
while(<LIST>) 
{
    ($drive) = unpack '@0 A3',$_;
    ++$count if m/Drive/;
    system("list -delete -dr -index ${count}");
}
[download]

This code can still made better, but that should get you off to a good start.