Square brackets [...] in a data structure like that is a reference to an array. A list (...) within a data structure like that is pretty meaningless as lists get flattened.
Either of the following two are the correct way of iterating through it:
my $aktData = $aktivitaet->{'activityData'};
foreach my $data (@$aktData) {
...
}
my @aktData = @{ $aktivitaet->{'activityData'} };
foreach my $data (@aktData) {
...
}
Edit: here's an example of why a list (...) within a hashref isn't what you want.
use Data::Dumper;
my $data = {
"list" => ( "foo", "bar", "baz" ),
};
print Dumper($data);