  The momentum distribution of $^{11}$Be fragments produced by the breakup of
$^{12}$Be interacting with a proton target at 700.5 MeV/$u$ energy has been
measured at GSI Darmstadt. To obtain the structure information on the anomaly
of the $N=8$ neutron shell, the momentum distribution of $^{11}$Be fragments
from the one-neutron knockout $^{12}$Be(p,pn) reaction, measured in inverse
kinematics, has been analysed in the distorted wave impulse approximation
(DWIA) based on a quasi-free scattering scenario. The DWIA analysis shows a
surprisingly strong contribution of the neutron $0d_{5/2}$ orbital in $^{12}$Be
to the transverse momentum distribution of the $^{11}$Be fragments. The
single-neutron $0d_{5/2}$ spectroscopic factor deduced from the present
knock-out data is 1.39(10), which is significantly larger than that deduced
recently from data of $^{12}$Be breakup on a carbon target. This result
provides a strong experimental evidence for the dominance of the neutron
$\nu(0d_{5/2})^2$ configuration in the ground state of $^{12}$Be.
