All measurements are only approximations.
No measurement is ever exact.
Using a ruler with 0.5cm marking, We might measure this eraser to be 5.5 cm long.
If we used a more precise ruler, with 0.1 cm markings, then we find the length to be 5.4 cm.
If we used a micrometer that measured to the nearest 0.01 cm, we may find that the measured length is 5.41 cm.
All measurements are approximations.
The Limit of Accuracy of a Measuring Instrument are +/- 0.5 of the unit shown on the instrument’s scale
The Perimeter of a shape is the distance around the shape. It is the sum of the lengths of the sides of the shape