In the sentence, the word citizens only appears once, and comes after upstanding. Upstanding means respectable, honest, etc which is somewhat close to perfect. So, one can infer that what continues after in the sentence is a description of being a perfect citizen.
|Picture by shingo|
The last part about sending a letter connects with how the 100% girl in real life was holding a letter. It also is another example of something that is standard.
So, this sentence is what answers the question "how can we define what a perfect citizen is?". A perfect citizen is one that can do standard, normal things. They do not have to be able to do any special acts, they just have to be normal. This "normal" changes according to where you are. An example of this is if one lives in a rural place, being able to grow crops might be normal; whereas if one lived in a city, it may not be the case.
Considering all this, the final answer for me would be: A perfect citizens is a citizen that can do things that are considered standard in where they live.