We introduce the perceptual decoherence functional Dp(rho, epsilon) = C(rho) - C(epsilon(rho)), defined as the difference between the l1-norm coherence of an input state rho and the coherence of its image under a quantum channel epsilon, both computed relative to a fixed reference basis. This work does not propose a new resource theory of coherence, nor a replacement for standard decoherence theory. Instead, it defines a state-dependent, observer-relative functional that quantifies how much coherence becomes inaccessible in the observer-accessible description induced by a given channel. We establish non-negativity for coherence non-generating (CNG) channels, monotonicity under channel composition, and an upper bound; derive the classical limit of Dp under a dephasing channel parameterized by a resolution parameter Lambda, showing it becomes analogous to mutual information in the fully dephased regime; compute Dp for canonical examples including single-qubit dephasing, amplitude damping, and partial-trace channels on entangled states; and position Dp explicitly relative to decohering power and Quantum Darwinism. Four falsifiable experimental predictions follow from the framework.