Interestingly, it was used as a measure of length, area, and volume from the times of ancient Rome. Likely, a Wikipedia entry suggests, the term was re-introduced to England during the Norman Conquest of 1066. Since that time, competing measurements have included rods, chains, furlongs, and miles. Unfortunately, few agreed on what the standards really were though. For example, Irish perches were considerably longer than English ones.
A perch was eventually standardized at about 30 1/4 square yards. Or, there are about 160 square perches to an acre. Finally, if you think metric, then consider a perch as 25.29 square meters. Yet just to throw cold water on your new knowledge, as it turns out, perches are an alternate name for rods in the context of measuring area. Nowadays, we take standardization pretty much for granted. Yet, I'd like to suggest that we still aren't as perfect as we believe. Here, waist sizes are still, of course, measured in inches. The problem is that not all clothing manufacturers agree on exactly what they are. I've noticed this situation especially since I've been shedding a lot of those extra pounds / kilograms / stones over the past year and a half.
If I want a suitable pair of Dickies work pants then I have to buy size 38's. Wrangler, the jeans maker, on the other hand, considers me a size 34! God bless 'em. I think they've got my future business ...