Sorry I didn't awnser the other half of your question.

$self is a ref to the Field structure. $self->$item is a ref to one of the keys in Field, dependant on the looping. $self->$item->$subItem is a ref to one of the keys in SubField for Field, again depending on the iteration of the loops.

This is a ref to the value in the tree at $subItem for $item for Level2 for Level1.

$tree->{Level1}->{Level2}->{$item}->{$subItem}

This is supose to act as a setter for $self. $self->$item->$subItem ( ) ;

Simply put, you get $subItem from tree and place it into $subItem of $self.