LTM log tmm errors "err tmm9[22715]: 012200001:3: TCL error"

I suspect the code within the switch statement is not being executed so the variable $detect_handshake is not set. Try initialising the variable at the start of the iRule, by placing the following before the switch statement

set detect_handshake 0

Is there other code you have not shared that sets this variable to ’1’?