hi! i've been experimenting around with the operators and i think i found something that is I think is in the right direction. I have 3 items on the goo canvas.

my $pixbuf = Gtk2::Gdk::Pixbuf->new_from_file("girl16.jpg");
    my $i1 = Goo::Canvas::Image->new($canvas->{'item_layer'},$pixbuf,-
+$pixbuf->get_width/2,-$pixbuf->get_height/2);
    $i1->set('operator'=>'over');

    $pixbuf = Gtk2::Gdk::Pixbuf->new_from_file("girl15.jpg");
    my $i2 = Goo::Canvas::Image->new($canvas->{'item_layer'},$pixbuf,-
+$pixbuf->get_width/2,-$pixbuf->get_height/2);
    $i2->set('operator'=>'over');

    $pixbuf = Gtk2::Gdk::Pixbuf->new_from_file("mask.png");
    my $i3 = Goo::Canvas::Image->new($canvas->{'item_layer'},$pixbuf,-
+$pixbuf->get_width/2,-$pixbuf->get_height/2);
    $i3->set('operator'=>'dest_atop');
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this codes gave me a black screen and part of girl15.jpg shows on the "black" area of the mask. which i think means that the masking works... but the only problem is that the "dest" has a color of black (according to cairo the default source color of a cairo context is black). so i thought that if i made the dest color transparent, maybe i will end up with girl16.jpg with a masked girl15.jpg on top. Problem is that i cant find a way to change the default color!