transparent layer over image

[Dmitriy Kuvshinov <dmitriyk genphys phys msu ru>]

Good day!

I have a picture received by a method  

$pixbuf = Gtk2::Gdk::Pixbuf->new_from_file ($img);

and show it.

But I want from above a picture to draw a line, not changing
thus contents of a variable $pixels  = $pixbuf->get_pixels();
(like Color Snooper example script, but without mouse movement).

How it can be made?

How i can set transparent color on top of image widget for
example "\x00\x00\x00" (the RGB colour pixel)?

I can create and manipulate an images from my data with help function of
pack(C*,$pix), where length of $pix is $w*$h*3 (for RGB).

Where i can find example about it?

Thanks!