Date::Calc offers the ability to find out if daylight saving is active at system time, but I didnt find a way to check this for a specific date.

It looks like you didn't read Date::Calc's perldoc very carefully.

In addition to the System_Clock funtion you describe, the functions Gmtime, Localtime, and Timezone all return a "$dst" value as well -- and unlike System_Clock, the other three all take an optional [time] parameter, to indicate which datetime you are interested in.