I saw it in English/Croatian online dictionary. Something to do with scribbling all the time, a person who scribbles, probably nonsense, but he writes it down nevertheless. Actually, I needed something similar, where a person writes often, but not necessarily rubbish.
It makes sense- the payment per line, but if it's 'liner', then it would refer to the person. I can't think of an idiom for what you're after, but would 'compulsive' do?
I think a 'penny-a-liner' would be a journalist who is paid by a newspaper by the number of words (or lines) he writes; thus, perhaps, one for whom quantity is more important than quality.
penny a liner 
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