As commented, perforce is a very old-fashioned word, so you might want to avoid it anyway. But another "usage note" that you probably won't find in dictionaries is that we don't often negate the term.
Here's an NGram where I multiplied occurrences of is perforce by 70 so we can see it on the same usage chart as is necessarily (which means exactly the same)...

...and here's the same NGram, but with both expressions negated...

Even when increased by a factor of 70, is not perforce practically "flatlines" against is not necessarily. So even if you don't want to avoid perforce completely, you should certainly consider avoiding it in "negating" contexts.
As regards the position, we normally put the adverb (perforce, necessarily, or similar) immediately after the copula verb is. It can be placed at the end of an utterance (as a "whole sentence adverb"), but that's not common.