Perforce means 'because there is no other choice'. It is an adverb, and a very old-fashioned, literary one — it sits in the sentence where 'necessarily' or 'of necessity' would sit, and marks that what follows was forced by circumstances, not chosen: 'The meeting was perforce a short one.' 'He must perforce agree.' You will meet it in nineteenth-century novels, in Shakespeare, in legal or academic writing where a writer wants an old formal flavour, and occasionally in a modern essay by someone showing off a little. Nobody speaks this word in ordinary life in Taiwan or anywhere else; if you used it in a conversation people would hear it as a joke or as very bookish. It is worth knowing so you can read it, and it is worth not using so you do not sound strange. Modern equivalents: 'inevitably', 'necessarily', 'we had no choice but to…'. Notice that it often comes between the subject and the verb, or right after 'must' — a position that itself feels old.

不得已地、必然地（古舊書面語，現代多說 necessarily／不得不）