When the configuration bit LVP is ON, the MCLR pin cannot be used as an input: MCLRE = OFF will not have any effect, the pin will always read as 0.

Furthermore, if LVP = OFF and MCLRE = OFF (very common case), you will need a programmer that is able to generate a "Vpp first" procedure that will be the only way to force the PIC to go into the programming mode. With a simplified programmer (there's a lot of on the net, wrongly claiming to be "PK2 clone") that uses a fixed permanent Vdd that cannot be switched off you will be stuck with a PIC that seems to act as a good ol' OTP!