So, is there an easy way to find out the time difference? Somehow Perl must already "know" this (otherwise, it would not be able to implement gmtime and localtime correctly, doesn't it?)

See gmtime, localtime, POSIX#tzset, ENV{TZ} and localtime? (inconsistent time in child script), Standard way to convert timezone in multithreaded script

If you OS is old, then localtime/gmtime/tzset.. might not work as expected since daylight savings time has changed a few times... there are probably other related caveats as well :) DateTime is better :)