Simple maths problem.

a_n is defined as 1 + (1/n).

S_n is defined as a₁ + a₂ + ... + a_n.

So we're gonna calculate S₁₅, and we're gonna use object-oriented programming because I'm me.

use v5.16;

package Local::App {
    use Zydeco;
    use List::Util 'sum';

    class Calc {
        method a ( PositiveInt $n ) = 1 + (1/$n);
        method S ( PositiveInt $n ) = sum( map $self->a($_), 1 .. $n )
+;
    }
}

my $calc = Local::App->new_calc;
say $calc->S(15);
[download]

Or, using rational numbers:

use v5.16;

package Local::App {
    use Zydeco;
    use List::Util 'sum';

    class Calc {
        method r ( $n ) = 1/$n;
        method a ( PositiveInt $n ) = 1 + $self->r($n);
        method S ( PositiveInt $n ) = sum( map $self->a($_), 1 .. $n )
+;
        
        class +Rational {
            use Math::BigRat;
            method r ( $n ) = Math::BigRat->new("1/$n");
        }
    }
}

my $calc = Local::App->new_calc_rational;
say $calc->S(15);
[download]