I think perhaps your confusion stems from thinking that there is only one variable called $n. In fact, each call to your function creates a brand new $n variable.

Study this. Note that I never add anything to $n - only subtract from it. Now run it to see the output:

use strict;
use warnings;

sub counter
{
    my ($n) = @_;
    
    print "N is $n\n";
    
    if ($n > 0) {
        counter($n - 1);
    }

    print "N is $n\n";
}

counter(4);
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