The slope is undefined.
Work Step by Step
A horizontal line has a slope of $0$ because it does not have a rise ($y$ value), but it does have a run ($x$ value). A line that is perpendicular has to be a vertical line. A vertical line has an undefined slope because it has a rise ($y$ value), but no run ($x$ value). Any fraction with $0$ as its denominator is undefined. Therefore, the slope of a line perpendicular to $l$ is vertical and has an undefined slope.