James Cutsinger on The Perennial Philosophy
James Cutsinger is a professor of Theology and Religious Thought at the University of South Carolina. This is a longer interview in which he "talks around" a lot of different topics regarding the Perennial Philosophy. I don't know that he ever gives a hard, propositional definition, but this is a really good introduction to the ideas that come out of the Perennial Tradition.
Eknath Easwaran: A Short Definition of the Perennial Philosophy
This definition of the Perennial Philosophy uses Hindu concepts (i.e. "The Atman"). Definitions will differ slightly and even Easwaran changes these propositions in some of his works. In his most famous set of translations, he defines the Perennial Philosophy as follows:
"(1) There is an infinite, changeless reality beneath the world of change;
(2) this same reality lies at the core of every human personality;
(3) the purpose of life is to discover this reality experientially; that is, to realize God while here on earth."
Shinzen Young on World Mysticism
This is Shinzen Young giving a somewhat winding talk about world mysticism from a Buddhist perspective. I find his contrast (at 9:00 and following) between pseudo-mysticism – the experience of "weird stuff" (gods, ghosts, ancestors, acquisition of powers, etc.) – and Mysticism with a Capital M – which he defines as "touching the Formless Source" – to be helpful in understanding what people mean when using the term.
In the Beginning was Consciousness
Although he doesn't use the term "Perennial Philosophy," the worldview presented in this lecture is representative of the Perennial Tradition. This is a fascinating lecture regardless of your religious viewpoint; it's worth listening to to whole thing if you have the time.