All you need to use is the auto-increment operator.

$a = 'A000001';

while (1) {
    print "$a\n";
    $a++;
}
[download]

look up the ++ operator in perlop under Auto-increment.

Also you should avoid the <> operator for <STDIN> unless you want the side-effects. merlyn explains it better than I.

UPDATE: If you want, take a look at a fun use of auto-increment.

OR

$a = 'A000000';
while (1) { print $a++,"\n"; }
[download]

grep

grep> cd pub grep> more beer