There could be a couple things wrong. One, just like your rain gauge that gets water in it, your dryer pipe may have gotten some snow sticking to the inside of the pipe. When the dryer came on the heat melted the snow. Shouldn't be a lot of water and it should stop very soon. Two, whoever installed the metal flange around the pipe where it exists the roof did a poor job. Water could be entering either under it or right around the pipe where it meets the flange. That should be caulked. Until you can get at it remove the snow around that area out about 3 feet or so. Remember, water inside the house can cause all kinds of damage, mold included, so don't take this lightly have it repaired asap. Hope that helps.