The default hardware configuration of the user interface block implies that:
There is no hardware to support debounce on the RST pushbutton: R36 is not mounted, so C4 has no effect. This was done to support the aWire programming and debugging interface (refer to Section Programming and Debug Interface). To enable the hardware debounce support on the RST pushbutton, mount the 0Ω R36 resistor (solder patch).
To locate the R36 resistor mentioned above, use the assembly top/bottom views provided within the AT32UC3L-EK schematics package.