The perspective correction tool is used to fix keystone problems in buildings and other objects which should be square and which are not. It places a grid over the image and you stretch it to align with the crooked sides. There are two settings - Forward and Backward and you will use Backward (Corrective) to fix the problem. When you have the grid in place, press Enter to make the correction.

In terms of photo editing and fixing there are plenty of tools to choose from. Unusually, the brightness and contrast, colour and saturation tools are all grouped together in the Tools, Color tools menu. These tools include Color balance, Hue-Saturation, Colorize, Brightness-Contrast, Threshold, Levels, Curves and Posterize.