A "composite" figure is just a plain shape with a chunk added or bitten out — an L-shape is the classic one. The perimeter rule hasn't changed (add every side), but two things make it sneaky: some side lengths aren't written on, and there's a line that looks like a side but isn't.
Trap One: the Missing Sides
An L-shape only labels some of its edges. You have to deduce the rest, and there's a neat rule: the bits along the top must add up to the full width along the bottom, and the bits up the side must add up to the full height. So a missing piece is just the long side − the bit you do know. In the toy, hit "work out the missing sides" and you'll see each "?" filled in by a subtraction.
Trap Two: the Line That Isn't a Side
It's tempting to slice the L into two rectangles with a line straight down the middle — that's a great move for area, but for perimeter that line is a fake. It's inside the shape; the ant never walks it. Press "show the trap line" to see it appear, dashed and labelled "not a side". Add only the edges on the real outline, never that internal cut.
Putting It Together
So the method for any L-shape is: fill in the missing sides by subtraction, then walk the outline once and add every real edge — and only those. Do that and a "composite figure" is no scarier than a square.