Two solutions come to mind for perimeter fence detection with CCTV.
(1) Fixed cameras with Motion Detection. (Not my preferred option)
The number of cameras required, placement and lens selection (vertical angles of view) are critical in the design phase to ensure that the entire fence line is covered. Often there will be vulnerable areas along the fence unless you overlap the field of view of each of the cameras, for example the area directly underneath and adjacent to the camera pole. Then the nuisance alarm rate of outdoor motion detection needs to be considered. What is acceptable to your customer?
(2) PTZ cameras with alarm presets and fence alarm devices (My preferred option)
There are a couple of fence alarm systems that offer detection zones, acoustic, such as E-flex / Guardwire (ok for chain mesh fences) and Fibre optics, such as Future Fibre Technologies (best for most types of fences). These systems often have a low nuisance alarm rate. The alarm detection zones are then set up with the presets on the PTZ cameras to look at the area of fence in alarm.
If you have chosen PTZ cameras then you will need to look at option 2, of course you could also use outdoor beams inside the fence line instead of fence alarms, my preference is a purpose designed fence alarm system particularly the fibre optics.