If the (Java) link you pointed to is actually the documentation for your Perl classes then this page gives you a list of methods that can be called on the ArtifactsInPlanningFolderSoapRow object. No harm in trying it - something like this:

# $artfList - a hash reference

my $array = $artfList->{dataRows};

for my $row_object (@$array) {
    my $id = $row_object->getId;
    my $category = $row_object->getCategory;
    print "ID: $id, Category: $category\n"; 
}
[download]

Update: if the above works this is how you could get the list of IDs:

my @ids;
for my $row_object (@$array) {
    push( @ids, $row_object->getId );
}
[download]

Or if you want to be fancy and do it all in one line:

my @ids = map { $_->getId } @{ $artfList->{dataRows} };
[download]