I don't think it's a good idea to open and close the file to write to each time. I'd initialize a hash with the year as key and the handle as value, opening a new one if none exists in the hash for that particular year. To keep things clean I'd use IO::File.

Also, but this is minor, I'd not use the substr since this is waste of time: $line =~ /^[0-9]{9}/ should do nicely as condition to keep the line.

Just my 2 cents, -gjb-