Here’s a few ideas.
The first is to to make the L shape out of a single object so it’s easy to move and rotate. Then when it lands you create the smaller squares on top of the shape, and then destroy the shape. You could utilize image points to mark where to create the smaller sprites.
Another idea is to have another hidden object to act as the rotation center and use the pin behavior to attach the squares. Delays of when the behavior runs may or may not be an issue.
A third idea is a more manual method. First you create the squares in the shape you want, and probably use an instance variable to help with picking just them and then rotate them with this equation:
Set position to (self.x-centerx)*cos(90)-(self.y-centery)*sin(90)+centerx, (self.x-centerx)*sin(90)+(self.y-centery)*cos(90)+centery
Where centerx, centery is the position to rotate around and 90 is how much to rotate.