In this paper, we prove that the system of generalized eigenvectors of the perturbed operator T (ε) := T 0 + εT 1 + ε 2 T 2 + · · · + ε k T k + · · · , forms an unconditional basis with parentheses in a separable Hilbert space X; where ε ∈ C, T 0 is a closed densely defined linear operator on X with domain D(T 0 ), having compact resolvent, while T 1 , T 2 , . . . are linear operators on X, with the same domain D ⊃ D(T 0 ), satisfying a specific growing inequality. An application to a problem of radiation of a vibrating structure in a light fluid is presented.