To your questions:

1. Is there a Tk::Zinc(::Graphics) way of building multiple items with similar characteristics? (and how good is my hacked solution)

Yes, look at the clone() method. Look in the zinc-demos, under User Contributed Demos, for TripleRotatingWheel. I make one wheel object, then clone it, then translate it into position. Like: my $arc2 = $zinc->clone($arc1);

2.Is there a reliable way to position text at center of group?

Here is a snippet to show you how. I also throw in a useful trick for reversing the y-axis direction, so coords are more like standard cartesian.

#!/usr/bin/perl
use warnings;
use strict;
use Tk; 
use Tk::Zinc;

my $mw = MainWindow->new;

my $width = 700;
my $height = 600;

$mw->geometry($width.'x'.$height);

my $zinc = $mw->Zinc(-width => $width, -height => $height,
                -backcolor => 'black',
        -borderwidth => 3, 
        -relief => 'sunken',
        )->pack;

$zinc->fontCreate(
    "fonta",
    -family => 'arial',
    -size   => 30,
    -weight => 'normal'
);


#create a group with it's origin at center
my $centergroup= $zinc->add('group',1,-visible=> 1);
$zinc->scale($centergroup,1,-1); #reverse direction of y axis
$zinc->translate($centergroup,$width/2,$height/2);


# Then we create a gray filled rectangle, in which we will display exp
+lain text.
$zinc->add('rectangle', $centergroup , [-100, -100, 100, 100],
       -linewidth => 2,
       -filled => 1,
       -fillcolor => 'SkyBlue',
       );

my $text = $zinc->add('text', $centergroup,
              -position => [0,0],
              -text => 'foobaz',
              -font => 'fonta',
              -anchor => 'center',
              -priority => 2 
              );


MainLoop;
[download]

3. Is there a way to correctly calulate delta x/y if scaling is active?

Your dragging works fine by itself, and your scaling works fine by itself. Maybe just make them mutually exclusive, and pop a warning saying scaling is disabled during dragging? How often is someone going to want to do that anyways? I saw a similar problem in the Gnome2::Canvas and Goo::Canvas where I needed to do some clever multiplications/divisions to account for the current scale when saving to pdf. All I can say is I spent a few hours experimenting before I got it right. :-)

4. Did I missing something when it comes to calculating positions of items relative to display settings?

If you are referring to the drag/scale problem, what I think you will need to do is have 2 separate code blocks for calculating dx/dy. One if scaling is active, and 1 for simple dragging. Since your scaling factor can constantly be changing while the darg is occurring, I think it will mean alot of recalculations for each pixel of drag. Think about it....how can Zinc know what your dx/dy is, if the scale is changing while you drag. At best, it will be jerky( visually not codewise :-) ). I have 2 suggestions. One is to pop a warning, saying scaling is disabled during a drag. Or two, get rid of the +/- button press for scaling, and use a SpinBox instead, to set the scale. That way, the mouse can only be in 1 place at a time...either setting the scale-SpinBox or dragging an item.