The only writing utensil I know is penitentiary
This can have a double meaning. Also note the potential wordplay.
penitentiary (Pen- in- it’s- entry)
He is saying that The pen is the only tool he has. But if you dig deeper he could be metaphorically saying how alot of youth in modern America are destined for failure and jail. A&E has a show that covers troubled youth having a dose of reality behind bars
In the pen you also write on walls. Just a typical thing to do to pass the time
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