++ELISHEVA ... good job!

My program is now consistently displaying little red curved push pins. :-)

Your math skills are quite impressive.

So the next step will be to calculate the correct terminal angle, in order to draw the arrowhead.

By the way, where you've written:

  if ($x2 < $x) {
    my $tmp = $x;
    $x=$x2;
    $x2=$tmp;
  }
[download]

the following is a bit more succinct (and avoids the temporary variable):

  ($x2 < $x) and ($x, $x2) = ($x2, $x);  # Swap $x and $x2
[download]