Perowne's quoting John Calvin in Psalm 56 In Perowne's commentary on the Psalms in one volume, he quotes Calvin  on page 445 in his treatment of Psalm 56.


  "Assuredly," says Calvin, "this is the true test and proof of our faith; when fears harass us, so far as our fleshly nature is concerned (pro sensu carnis), but do not overthrow and unsettle our minds. It seems, indeed, as if fear and hope were feelings too contrary the one to the other to dwell in the same heart; but experience shows that Hope there in fact really reigns where some portion of the heart is possessed by Fear. . . ."


He goes on for a few more sentences. Where in Calvin's writings did he in fact write this? Thanks!
This is from Calvin's treatment of Psalm 56:3.  The most common translation of this work, by James Anderson (published 1846), is available online for free, and the relevant section reads:


  The true proof of faith consists in this, that when we feel the solicitations of natural fear, we can resist them, and prevent them from obtaining an undue ascendancy. Fear and hope may seem opposite and incompatible affections, yet it is proved by observation, that the latter never comes into full sway unless there exists some measure of the former. (CCEL; cf. Google Books)


Comparing your quote to this one, it's easy to see how they differ only in translation.  Since his exact wording doesn't appear to predate his work, it seems likely that Perowne provides his own translation of Calvin's original text.