my $bf_1;
{ # Start a new block to avoid cluttering the environment
    my $sum_x = 0;
    $bf_1 = sub {
        # This block references $sum_x,
        # so it will be captured in the closure
        my ($operation, @args) = @_;
        # You didn't mean @ARGV; it's for command-line args
        if($operation eq 'Add') {
            $sum_x += $args[0];
        }
    };
}