There are several ways to get the data. first_child only goes down 1 step in the tree. To get the text, you would need $drug->first_child('groups')->field( 'group'), or $drug->next_elt( 'groups')->text or $drug->findvalue( 'groups/group'). INTEREST1 is an attribute, so you would get it with $drug->att( 'type').
There is probably a problem though either with your simplified data or with the code. You have te tags in your example as data1 and data2. If this is really the case, then you won't access them with $root->children('data'), since the tags are NOT data. So either the tags are data, or you have to get them using $root->children.