$top = [$x_t,$y_t,$z_t]; # you got this from your maxima search
@subset = ( [$x0,$y0,$z0], [$x1,$y1,$z1], ...); # the subset in step1
@contour = sort { 
  atan2($a->[1] - $top->[1], $a->[0] - $top->[0]) 
  <=> 
  atan2($b->[1] - $top->[1], $b->[0] - $top->[0]) 
} @subset;