Unrelated to your scoping issue (once again it's been answered well by others before this) but there's a Perl idiom for this tidbit of code:

my $config_file = shift;
        if (!$config_file) {
                $config_file = "etc/allweb-backup.conf";
        }
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which is a simple: my $config_file = shift || "etc/allweb-backup.conf"; Same results, cleaner code (IMHO).