The photoluminescence in a lead halide perovskite is measured for different temperatures (5–300 K) and excitation fluences (21–1615 μJ cm−2). It is found that amplified spontaneous emission (ASE) is observed for an excitation density larger than about 1 × 1018 cm−3 for both the tetragonal phase above 163 K and the orthorhombic phase below about 163 K. The fluence that is required to obtain this excitation density depends on temperature and phase since the nonradiative decay of excitations is temperature activated with different activation energies of inline image and inline image for the tetragonal and orthorhombic phase, respectively. The ASE from the tetragonal phase—usually prevailing at temperatures above about 163 K—can also be observed at 5 K, in addition to the ASE from the orthorhombic phase, when the sample is previously exposed to a fluence exceeding 630 μJ cm−2 at a photon energy of 3.68 eV. This additional ASE can be removed by mild heating to 35 K or optically, by exposing the sample by typically a few seconds with a fluence around 630 μJ cm−2. The physical mechanism underlying this optically induced phase transition process is discussed. It is demonstrated that this phase change can, in principle, be used for an all-optical “write–read–erase” memory device.