The sinner is determined to evil in general, not to each act: Perkins's prisoner
Thesis XL. Add that man in the state of sin, left to himself, is indeed determined to evil in general, but not to each particular evil action: he can abstain from this one or that one, just as a man locked in prison can walk about freely inside it. William Perkins uses this comparison to illustrate the point.
'I will show by an example,' he says, 'that some things are done necessarily and freely at the same time. Suppose a man has been thrown into prison, so that he must stay there and cannot leave for wherever he wants. He can still move about and walk freely within the prison. It is the same with the human will. Bound by the chains of sin, it naturally does nothing but sin, and so sins necessarily; yet it sins of its own free will' (The Reformed Catholic, first controversy, On free will, fourth conclusion).
Latin
XL. Adde quod homo in statu peccati sibi relictus, in genere quidem ad male agendum determinatus est, non tamen ad singulas malas actiones est determinatus, sed potest ab hac vel illa abstinere: sicut homo carcere conclusus potest per carcerem libere discurrere. Qua similitudine ad hanc rem illustrandam utitur Gulielmus Perkinsus, Nam, inquit, esse quasdam res, quae necessario & libere simul fiant, exemplo demonstrabo. Sit aliquis in carcerem detrusus, cui necessario ibi sit manendum, quique non possit exire quo velit. Is tamen talis movere se potest, ac libere deambulare intra ipsum carcerem. Similiter se res habet cum voluntate humana, quae licet peccati vinculis constricta, naturaliter aliud non faciat quam ut peccet, atque ita peccet necessario: tamen eadem peccat propria ac libera voluntate. In Catholico Reformato, Controversia prima de libero arbitrio Conclusione quarta.