perry is slightly different to cider : cider is an alcoholic fermented beverage made from apple juice, and any apples can go into cider, and other things as well, like pears. Perry is made from only pears, and only Perry pears (like cider apples rather than dessert, or cooking, or eating apples) - so if you make a fermented beverage just from dessert pears, you can't call it Perry, it's Pear cider. Mostly pears, with some apples, will be pear cider.
pears are harder to extract juice, so what usually happens with apples is that they're milled and then pressed fairly soon afterwards, with pears they are milled/scratted, and then left to maccerate for 24 hours before being pressed - which extracts a lot more juice.
hope that helps
