Thanks again Shawn for all your help. I’ve uploaded my perl file to github, and I think I’ve covered off all the attribution. The only slight difference I see so far with this code and the python code is that sometimes WU have multiple forecasts per hour. The python grabs the cloud cover for the first forecast, and the perl grabs the last one. This is really minor, and doesn’t really change the outputs.
https://github.com/hplato/misterhouse/blob/eto/code/common/calc_eto.pl