You can view WebGUI's API documentation in a number of ways.

1. From the command line you can type perldoc <filename> and the documentation will be displayed to your screen.
2. You can view it on the web by visiting http://www.plainblack.com/downloads/builds/VERSION/api/ at any time. For example: http://www.plainblack.com/downloads/builds/7.7.24-stable/api/. You can find all builds by visiting http://www.plainblack.com/downloads/builds/.
3. And finally, starting with WRE 0.8.0 a documentation server is included that allows you to view and search the API.