  A prototype for the instrumented decay tunnel of ENUBET was tested in 2018 at
the CERN East Area facility with charged particles up to 5 GeV. This detector
is a longitudinal sampling calorimeter with lateral scintillation light
readout. The calorimeter was equipped by an additional "$t_0$-layer" for timing
and photon discrimination. The performance of this detector in terms of
electron energy resolution, linearity, response to muons and hadron showers are
presented in this paper and compared with simulation. The $t_0$-layer was
studied both in standalone mode using pion charge exchange and in combined mode
with the calorimeter to assess the light yield and the 1 mip/2 mip separation
capability. We demonstrate that this system fulfills the requirements for
neutrino physics applications and discuss performance and additional
improvements.
