Benedictine Father John Main often used John Cassian's expressions pax perniciosa and sopor letalis, but he did not use them in same senses as Cassian.
Here are a couple of examples of John Main's usage from his published talks.
From The Gethsemani Talks:
The temptation is to return to that prayer we might describe as the prayer of anaesthetised, floating piety -- the prayer that John Cassian termed the "pax perniciosa" (the ruinous peace) and the "sopor letalis" (the lethal sleep).
From Letters from the Heart:
One of the great perils of this pilgrimage is that we talk about it so much and so cleverly imagine ourselves on it that we actually fail to tread it, to put one foot in front of the other. I have spoken to you of this danger often enough. It is the pax perniciosa, mere religiosity or "body floating."
In the first quote, John Main is clearly equating the pax perniciosa and the sopor letalis.
But let's look at John Cassian's use of sopor letalis. This passage is from conference 10, chapter 8, as given in Migne's Patrologia Latina, vol. 49, cols. 829-830:
Euenit namque, ut cum de theoriis spiritalibus euagati ad nosmet ipsos uelut de letali sopore conuertimur et tamquam expergefacti materiam quaerimus, qua illam quae obruta est spiritalem memoriam resuscitare possimus, retardati ipsius inquisitionis mora, priusquam repperiamus eam, a nostro conatu iterum deuoluamur, et antequam spiritalis quidam pariatur intuitus, concepta cordis euanescat intentio. Quam confusionem idcirco nobis accidere satis certum est, quia speciale aliquid prae oculis propositum uelut formulam quandam stabiliter non tenemus, ad quam possit uagus animus post multos anfractus ac discursus uarios reuocari et post longa naufragia uelut portum quietis intrare.
In the English translation of Edgar C.S. Gibson (1894):
For it happens that when we have wandered away from our spiritual speculations and have come back to ourselves as if waking from a deadly sleep, and, being thoroughly roused, look for the subject matter, by which we may be able to revive that spiritual recollection which has been destroyed, we are hindered by the delay of the actual search before we find it, and are once more drawn aside from our endeavour, and before the spiritual insight is brought about, the purpose of heart which had been conceived, has disappeared. And this trouble is certain to happen to us for this reason because we do not keep something special firmly set before our eyes like some principle to which the wandering thoughts may be recalled after many digressions and varied excursions; and, if I may use the expression, after long storms enter a quiet haven.
For Cassian, then, the sopor letalis is the phenomenon of drifting unawares into irrelevant thoughts. That's not at all the same as the "floating piety" of Father John Main.
Now let's look at the pax perniciosa. For John Main this is similar to what he understands by sopor letalis. But for Cassian, the pax perniciosa it is not primarily a state of mind but rather a moral condition. This next passage is not from the conferences on prayer (conferences 9 and 10) but rather from conference 4, chapter 7. From Migne's Patrologia Latina, vol. 49, cols. 591-593:
Hanc pugnam utiliter nostris quoque membris insertam, etiam in Apostolo ita legimus (ad Gal. V). Caro enim concupiscit adversus spiritum, spiritus autem adversus carnem. Haec autem invicem adversantur sibi, ut non quaecumque vultis, illa faciatis. Habes et hic pugnam invisceratam quodammodo corpori nostro, dispensatione Domini procurante. Quidquid enim generaliter et sine alicujus exceptione omnibus inest, quid aliud judicari potest, nisi ipsi humanae substantiae post ruinam primi hominis, velut naturaliter attributum, et quod universis congenitum concretumque deprehenditur, quomodo non credendum sit arbitrio Domini non nocentis, sed consulentis insertum? Causam vero hujus belli, id est, carnis et spiritus, hanc esse describit: Ut non, inquit, quaecumque vultis, illa faciatis. Ergo id quod procuravit Deus, ut non posset a nobis impleri, id est, ut non quaecumque volumus, faciamus, quid aliud si impleatur credi potest esse quam noxium? Et est quodammodo utilis haec pugna dispensatione Dei nobis inserta, et ad meliorem nos statum provocans atque compellens, qua sublata proculdubio pax econtrario perniciosa succedet.
In the English translation by Gibson:
This conflict too we read in the Apostle has for our good been placed in our members: "For the flesh lusteth against the spirit: and the spirit against the flesh. But these two are opposed to each other so that ye should not do what ye would" (Galatians 5:17). You have here too a contest as it were implanted in our bodies, by the action and arrangement of the Lord. For when a thing exists in everybody universally and without the slightest exception, what else can you think about it except that it belongs to the substance of human nature, since the fall of the first man, as it were naturally: and when a thing is found to be congenital with everybody, and to grow with their growth, how can we help believing that it was implanted by the will of the Lord, not to injure them but to help them? But the reason of this conflict; viz., of flesh and spirit, he tells us is this: "that ye should not do what ye would." And so, if we fulfil what God arranged that we should not fulfil, i.e., that we should not do what we liked, how can we help believing that it is bad for us? And this conflict implanted in us by the arrangement of the Creator is in a way useful to us, and calls and urges us on to a higher state: and if it ceased, most surely there would ensue on the other hand a peace that is fraught with danger.
So for Cassian, the pax perniciosa is the apparently pleasant condition of existing without moral conflict.
I wonder if this pax perniciosa is similar to the thoughtless "non-reflective consciousness" that Phil mentioned in the context of Daniel Helminiak's discussion of Bernard Lonergan (though I think it actually originates in Jean-Paul Sartre's Being and Nothingness).
And that leads me to wonder if, by the pax perniciosa, John Cassian meant something similar to the moral wasteland of Eastern gurus. Having dismissed morality as mere social conditioning, Eastern gurus are free to act out their hostile, sexual, and sadistic urges without compunction. And this casting off of ethics would explain the harmful behavior of any number of toxic spiritual teachers -- Rajneesh a.k.a. Osho, Chogyam Trungpa, etc. etc. etc.