Digital Pins reads 1 when configured with internal_pullup
There is no difference between a switch and a "pin" for input. All are either HIGH or LOW. You tie it HIGH with the pullup, and then try giving it a HIGH with +5V (Caution: The Due is a 3.3V device, not 5V!). 

At no point do you ever give it a LOW signal (connect the input to GND), so how can it ever read LOW?