kennethk, thank you very much! A little more tweaking, at now it works. I'm posting the final function (just for reference for others seeking like me): 
sub get_descriptions {
    my ($self, $packages) = @_;

    my @descriptions = ();
    for my $class (@{$packages->{classes}}) {
           my %local_hash = ();
        $local_hash{id} = $class->{classid};
          $local_hash{description} = $class->{classdescription};
          push @descriptions, {%local_hash};

           for my $method (@{$class->{methods}}) {
               my %local_hash = ();
               $local_hash{id} = $method->{methodid};
               $local_hash{description} = $method->{methoddescription}
+;
               push @descriptions, {%local_hash};
           }
           for my $attribute (@{$class->{attributes}}) {
               my %local_hash = ();
               $local_hash{id} = $attribute->{attributeid};
               $local_hash{description} = $attribute->{attributedescri
+ption};
               push @descriptions, {%local_hash};
           }
    }
    
    @descriptions;
}

[download]

For your note: I agree for the example the arrays are not neccessary, but there can be multiple methods or attributes (it's an UML XMI generator). As far as I understood the code, there are no accessor methods (the already existing part of the generator retrieves its data in the same way).

Thank you again, this helps me a lot!

Niko

PS: Is there a good reference where I can learn the subtleties of scalars, arrays and hashes? I understand the basic difference, but I'm totally at a loss when to use which prefix (e. g. my %local_hash = (); vs. $local_hash{id} vs. $class->{methods}).

In reply to Re^2: Retrieving data from deep structure by enikao
in thread Retrieving data from deep structure by enikao

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