Let $\rho$ be a modular function satisfying a $\Delta_2$-type condition and $L_\rho$ the corresponding modular space. The main result in this paper states that if $C$ is a $\rho$-bounded and $\rho$-a.e sequentially compact subset of $L_\rho$ and $ T: C\rightarrow C$ is an asymptotically regular mapping such that $\displaystyle \liminf_{n \rightarrow \infty}|T^n| < 2 $, where $|S|$ denotes the Lipschitz constant of $S$, then $T$ has a fixed point. We show that the estimate $\displaystyle \liminf_{n \rightarrow \infty}|T^n| < 2 $ cannot be, in general, improved.