firewall: Fix perl coding error.

Example:
	my @as = (1, 2, 3);
	foreach my $a (@as) {
		$a += 1;
		print "$a\n";
	}

$a will be a reference to the number in the array and not
copied. Therefore $a += 1 will change the numbers in the
array as well, so that after the loop the content of @as
would be (2, 3, 4).
To avoid that, the number needs to be copied into a new
variable like: my $b = $a; and we are fine.

This caused that the content of the @sources and @destinations
array has been altered for the second run of the loop and
incorrect (i.e. no) rules were created.