$spans_ref is an arrayref of x,y,width horizontal lines. If $sorted is true then they're assumed to be sorted by increasing y coordinate (allowing a single pass across the region rectangles). $function is called

&$function ($x, $y, $width, $data)

for each portion of a span which intersects $region. $function must not change $region.

$region->spans_intersect_foreach ([ 0,0,50, 20,20,100, 0,10,50 ],
                                  0, # spans not sorted by y
                                  \&my_callback,
                                  'hello');  # userdata
sub my_callback {
  my ($x, $y, $width, $userdata) = @_;
  print "$userdata: $x, $y, $width\n";
}