Maybe something like this would work:
0->B
For a,28,63
B+1->B
Pxlline A,44,A,44+B
Pxlline A,44,A,44-B
Endfor
This works on my Ti-89 basic. If you edit it slightly it'll probably work for you.

EDIT: I made a mistake(also x and y are switched I think)
Better code:
0->B
For a,0,95
B+1->B
Pxlline 28,44,63,B(You might have to swap the x with y so it's 44,28,B,63)
Endfor


If the beam doesn't need to be rotated, the code posted above should work well (although since the lines are vertical, I think the Rect() command may be faster).  Does your flashlight beam need to be rotated?  Because that completely changes the challenge

Yes it does need to be rotated (Just a test, not a game )
But wouldn't Rect( do a rectangle and not a "pyramid"-like shape?

You can for-loop the rectangular boundary of the region you want to illuminate (could be the whole screen) and then for each pixel, calculate its angle to the light source.  Only turn on the pixel if the angle is between the 2 angles of the light.  You can calculate the angle from pixel (x,y) to light source (a,b) with tan^-1(x-a,y-b)

You can't.  In your original code you only had vertical lines, which I was substituting for a 1 wide rectangle for speed If the beam is rotated, things become a bit more complicated.  Quigibo's solution would work, but I fear for the speed, since you would be doing a tan-1 for every pixel on the screen, which is likely going to be super slow.  I imagine there would also be other options involving custom line algorithms and rectangular scan-lines, but this does seem like a tricky problem

It's still an option, but if you are doing vertical or horizontal lines, you might as well use Rect().