And yes, I'm aware it can be solved without object-oriented programming.

use v5.16;

package Local::App2 {
    use List::Util 'sum';
    use Exporter::Shiny 'S';
    
    sub a {
        my $n = shift;
        1 + (1/$n);
    }
    sub S {
        my $n = shift;
        sum( map a($_), 1 .. $n );
    }
}

use Local::App2 'S';
say S(15);
[download]

I like using OOP though because of the ease with which parts can be overridden, like in the rational number version.