  We present an ab initio simulation of $90^{\circ}$ ferroelastic twins that
were recently observed in methyl ammonium lead iodide. There are two
inequivalent types of $90^{\circ}$ walls that we calculate to act as either
electron or hole sinks which suggests a possible route to enhancing charge
carrier separation in photovoltaic devices. Despite separating non-polar
domains, we show these walls to have a substantial in-plane polarisation of
$\sim 6 \phantom{|} \mu \text{C}\phantom{|}\text{cm}^{-2}$, due in part to
flexoelectricity. We suggest this in turn could allow for the photoferroic
effect and create efficient pathways for photocurrents within the wall.
