Many thanks for the useful pointers.

For now, I've opted for the simplest way out and have built something that lets me manipulate a standard window; I can use it to 'block out' the area of interest on the desktop and it returns the X11-style attributes of the window. Code follows:-

# --------------------------------------------------------------------
+-------
#  xrselw32.pl - Use a resizable window to define a desktop monitoring
+ area
#  jjg, 5-Aug-2016
#
#  Description:
#    A quick'n'dirty tool to allow me to use a standard 
#    Windows resizable 'Window' to block-out a region 
#    of the desktop and return the location in an X11 
#    'WxH+X+Y' format.
#
#  Notes:
#    1.  I can't work-out how to place buttons and other 
#        objects within the window (there doesn't seem to be 
#        any Tk-like concept of 'pack' or similar in Win32::GUI), 
#        so I took the KISS approach and have left the system 
#        menu(s) on the main window;  this allows me to use a 
#        single button to accept the window and use the 
#        "system menu" and '[X]' window button to act as a 
#        'Cancel' button.
#        
#    2.  I also can't work-out how to properly communicate 
#        'outside' of the Dialog 'monitoring' thing other than 
#        through the use of global variables = kludge.  Hence, 
#        I'll make this whole silly thing an external tool, much 
#        the same as I do with the linux version.  Inefficient 
#        but Pffft.
#        
#    3.  This is something that works Ok, so I'm not 
#        particularly worried that it's not as pretty as 
#        it might be... 
#        
#  References:
#    http://www.perlmonks.com/?node_id=1169110 (my question)
#    http://search.cpan.org/~kmx/Win32-GUI-1.13/GENERATED/Win32/GUI/Tu
+torial/Part1.pod
#
# --------------------------------------------------------------------
+-------

use Win32::GUI();

Get_Script_Name(\$my_proc, \$my_path);  # Determine the name of this s
+cript

$W_INIT = 440;    # Initial window size
$H_INIT = 330;

$main_window;     # Global declaritive ('Wot!?')

$W, $H, $X, $Y;        # GLobals that define the capture area: WxH+X+Y
$position_status = 0;  # ...and their validity

Place_Window();   # Get the window position

if ($position_status) {
   printf("%dx%d+%d+%d", $W, $H, $X, $Y);
   
} else {
   printf("");

}

# ==================== end mainline ====================
[download]

Read more code in this 'spoiler', if you want...

Ultimately, I still hope to work out how to draw a simple outlined rectangle... but this script works Ok.

Again, many thanks for the assistance.

John