class Student:
    int grade

    boolean IsStudentPassing():
        return grade >= MIN_PASSING_GRADE


...

// In another file
constant int MIN_PASSING_GRADE = 70

ensure that IsStudentPassing is false when grade is MIN_PASSING_GRADE - 1
ensure that IsStudentPassing is true when grade is MIN_PASSING_GRADE

ensure that IsStudentPassing is false when grade is 69
ensure that IsStudentPassing is true when grade is 70

use MooseX::Declare;
class Student {
    has grade => (
        is => 'ro', isa => 'Num', required => 1,
    );

    method min_passing_grade {
        return MIN_PASSING_GRADE;
    )

    method is_student_passing () {
        return $self->grade >= $self->min_passing_grade
    }
}

class t::Student {
    use Test::Sweet;
    use Test::MockObject::Extends;

    test correctly_determines_student_is_passing () {
        my $student = $self->_make_student($self->_passing_grade);

        ok($student->is_student_passing);
    }

    method _make_student (Num $grade) {
        my $student = Test::MockObject::Extends->new(
            $student->new(grade => $grade)
        );
        # Here's the important line:
        $student->set_always(
            min_passing_grade => $self->_passing_grade
        );
        return $student;
    }

    method _passing_grade () { 40 }

    test correctly_determines_student_is_failing () {
        my $student = $self->_make_student($self->_passing_grade - 1);

        ok(not $student->is_student_passing);
    }
}