Buy a Dog or two ... and I don't mean a Pekinese or a Poodle either!

IR Beams can easilly go for 100m or more... but are tricky to set up... the professional one's are best because they have vernier adjustment on the mirrors... a fraction of a degree at 100m or more means you miss alignment on the target.

Try running a voltage through the razor wire (a few kV would do)... PIC can then detect if it's been cut.