“Perfect for what?” you might ask. The perfect climate for skiing is presumably different from the ideal climate for growing vine, sailing or building a solar power plant. Moreover, what’s perfect for me may not be perfect for you, as tastes differ in weather as it does in everything else. Some people may prefer cloudy weather, or large variation over the annual cycle, whereas others may like it dry and hot throughout the year.
Nevertheless, many people would probably agree that an ideal climate is one which is warm but not too hot, relatively dry but with some rainfall, and copious amounts of sunshine. So a vague definition of the perfect climate would be something like:
Pleasant t-shirt weather year-round, mainly sunny but with moderate rainfall, not too windy, not too hot and not too humid.
And if that’s not precise enough for you, then how about this:
Average minimum daily temperature in the coldest month: at least 15 ℃ (59 F)
Average maximum daily temperature in the warmest month: at most 30 ℃ (86 F)
Average daily sunshine, as a proportion of the maximum possible, in the least sunny month: at least 50%
Average precipitation in the dryest month: no less than 20mm (0.79 in)
Average precipitation in the wettest month: no more than 100mm (3.83 in)
Average wind speed in the windiest month: no more than moderate breeze (7.9 m/s = 28 km/h = 17 mph at most)
Though these are technical terms and the precise numbers can be debated, it seems that many people would roughly agree with this definition.
One could add a humidity criterion to the above. But if the average daily temperature does not exceed 30 ℃, then discomfort from humidity is unlikely to be a serious problem.
Though few (if any) places will qualify under this strict definition, there are some that are ‘almost perfect’. For instance, some places people typically associate with a perfect climate are often uncomfortably hot and humid for at least parts of the year. And other places have great weather all year except for a torrential rainy season.