DateTime automaticly use DST iif the date is within the DST-timespan?

Yes, I think so. See How does DST affect day length?

that is also a bit weird because not all CET-Countries may habe DST or am i wrong?

Don't know , don't care, its why I use DateTime. I think this is why you can specify timezone by country ( DateTime::TimeZone:: ) since DST is political concept and DateTime gets this information via olsondb

use DateTime;
my $tz = DateTime::TimeZone->new(qw{ name CET });
printf "%20s %s\n", $_, $tz->$_
    for qw/ is_floating is_utc has_dst_changes is_olson name category 
+/;
__END__
         is_floating 0
              is_utc 0
     has_dst_changes 54
            is_olson 1
                name CET
            category CET
[download]

Which is the CET you want?