There's a proposal due to the prolog-commons initiative for a dictionary-like object (that is also implemented in SWI Prolog, I believe).

And in Prolog you can, of course, just trivially use your own term structure for item-values:

    p([ item : "value", ... ])

You could even use JSON-like terms in Prolog (with maybe a little help by the op/3 directive to sort out parsing priorities):

    A_Prolog_Term = {
      x: y,
      z: [a, b, {1, d} ]
    }

But the more fundamental approach IMHO would be to use nested knowledge-base terms:

    p(
      q(whatever).
      r(X) :- q(X).
    ).

As pioneered in the 1980s for attribute grammars, using ^^ and other special graph tokens as nested "implies" operator, though (see "definite-clause translation grammars").