Cold bright winters and long nights provide the chance to fish through the ice. Quality perch are located (caught and released) using small jigs fished just off bottom. Fish one spot then move a few yards, drill another hole and fish again.
As long as the ice is a good 10cm thick it is quite safe. February/March is normally the best time of the year. The ice starts breaking up in April and by May it is all gone and it is back to conventional fishing methods.
http://anglersworld.tv/coarse-fishing/sweden/varmland-county/byalven-cottages/