We consider the Cauchy problem for an n x n strictly hyperbolic system of balance laws ut + f(u)x = g(x, u), x ∈ ℝ, t > 0, ||g(x, ·)||C² ≤ M̃ (x) ∈ L¹, endowed with the initial data u(0, .) = uo ∈ L¹ ∩ BV(ℝ; ℝn). Each characteristic field is assumed to be genuinely nonlinear or linearly degenerate and nonresonant with the source, i.e., |λi(u)| ≥ c > 0 for all i ∈ {1, . . . ,n}. Assuming that the L¹ norms of ||g(x, ·)||C¹ and ||uo||BV(ℝ) are small enough, we prove the existence and uniqueness of global entropy solutions of bounded total variation extending the result in [D. Amadori, L. Gosse, and G. Guerra, Arch. Ration. Mech. Anal., 162 (2002), pp. 327-366] to unbounded (in L∞) sources. Furthermore, we apply this result to the fluid flow in a pipe with discontinuous cross sectional area, showing existence and uniqueness of the underlying semigroup. [ABSTRACT FROM AUTHOR]