Your problem is that you have something like 10 LEDs, each wanting to pull 10-40 milliamps. Your Arduino output pin is only rated for about 40mA total and you really shouldn't hold it at that.
Fortunately there is an example in the playground...
The transistor at the bottom works rather like a switch, when the Arduino pin is high is turns on the transistor and lets the light shine.
Ordinarily you would need a current limiting resistor to keep from exploding your LED, but you seem to have verified that your flashlight won't pop at 5V.
Another note: I just guessed at the total current draw of your lamp, if I'm off by a factor of two then it will draw more than the Arduino supplies, so if you reboot when the light comes on... you know where to look. That can be handled if needed.