Interactions of solitons generated by pairs of optical pulses are studied using the perturbed nonlinear Schrödinger equation, which models nonlinear optical pulses in a monomode fiber. We examine the mechanism of the soliton interaction both analytically and numerically and show that a bound 2-soliton splits into individual solitons traveling with different speeds because of higher-order effects. The numerical results are in extremely close agreement with those obtained recently by Mitschke and Mollenauer in their experiments on 2-solitons in optical fibers [Opt. Lett. 12, 355 (1987)].