You need to declare the variable. Try:

my $counter = 0;

Why? He is properly declaring his variable to be a package global, and that is perfectly legit under strictures. It would be a mistake to try to also declare it to be a lexical variable, as that would create two separate variables.

His problem is that he initialized $counter before declaring it. Here is his code:

$counter = 0;
use vars qw($counter);
[download]

The proper sequence of events is to declare the variable, and then start using it (or initialize it), like this:

use vars qw($counter);
$counter = 0;
[download]

HTH.