In quadrilateral $ABCD, \angle B$ is a right angle, diagonal $\overline{AC}$ is perpendicular to $\overline{CD},$ $AB=18, BC=21,$ and $CD=14.$  Find the perimeter of $ABCD$.
1. **Calculate the length of diagonal \( AC \):**
   Since \(\angle B\) is a right angle, triangle \( \triangle ABC \) is a right triangle with \( AB \) and \( BC \) as the legs. We can use the Pythagorean theorem to find \( AC \):
   \[
   AC = \sqrt{AB^2 + BC^2} = \sqrt{18^2 + 21^2} = \sqrt{324 + 441} = \sqrt{765} = 3\sqrt{85}
   \]

2. **Calculate the length of \( AD \):**
   Given that \( AC \) is perpendicular to \( CD \), triangle \( \triangle ACD \) is also a right triangle with \( AC \) and \( CD \) as the legs. We can use the Pythagorean theorem again to find \( AD \):
   \[
   AD = \sqrt{AC^2 + CD^2} = \sqrt{(3\sqrt{85})^2 + 14^2} = \sqrt{9 \cdot 85 + 196} = \sqrt{765 + 196} = \sqrt{961} = 31
   \]

3. **Calculate the perimeter of quadrilateral \( ABCD \):**
   The perimeter \( P \) is the sum of the lengths of all sides:
   \[
   P_{ABCD} = AB + BC + CD + AD = 18 + 21 + 14 + 31 = 84
   \]

The final answer is \(\boxed{84}\).