My answer assumes that the starting point is the first and last item in the x and y vectors, so if that's not your case, you'll have to add the first point to the end.
x_diff = diff(x);
y_diff = diff(y);
segment_lengths = sqrt(x_diff.^2+y_diff.^2);
perimeter = sum(segment_lengths);
This could be cut down to one line of code, but I wrote it out so you can see each step. You calculate the differences in x and y between each point. The length of each line segment can be calculated according to the Pythagorean theorem (x_diff^2 + y_diff^2 = segment_length^2). The perimeter is the sum of the line segment lengths.
If you're curious, this is how the same code would look when condensed to one line:
perimeter = sum(sqrt(diff(x).^2+diff(y).^2));
Hope this helps!
