Hi,

A couple of IR-led with proper lenses and some receivers would make a simple fens. If you put them close enough lets say every 50m you can easily detect if anyone breaks the beam and connected with RS-485 you can indicate what section it is done.

Lasers would work too but are more difficult to modulate and would require better adjustments. A quite wide IR beam along the top of the fence and the receiover in a pipe to give som direction and shadow.

I guess landmines is not an option here