Living life through a pen
A pen is an object you would write with, in which you want to record and/or write down things — i.e. stories, information, dreams, aspirations, etc. They are living their life by writing (“through a pen”) about….
…What they’re going through while in a penitentiary. Pen is short for “penitentiary,” in this case. Literally, they’re living their life literally in a penitentiary (“through a pen”).
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