Respuesta
The perpendicular slope is the negative reciprocal: $m_{\perp} = -\frac{1}{m}$ (assuming $m \ne 0$).
Explicación
The product of the slopes of two perpendicular non-vertical lines is always $m_1 \cdot m_2 = -1$. Horizontal lines ($m = 0$) are perpendicular to vertical lines (undefined slope).