$widget -> geometry("wxh+x+y")

width (w), height (h), yposition (x), and yposition (y) can be set as string
using above syntax. Position and size can be set independently, e.g.
"+x+y"  or "100x200".
Without a parameter the actual size and position is returned in the above
syntax.
Attention: Values are "0" before the widget has manifested itself on the
screen; use
update() or waitVisibility() if unsure.

Note: All values are in pixels.

The screensize can be requested using
$top -> screenwidth()
and
$top -> screenheight()

Thus (e.g.):
$h = $top->screenheight()  - 40;
$w = $top->screenwidth() - 40;

$top ->  geometry("${w}x$h+20+20")

# if you specify -0-0 for position, you anchor in the lower right corner 

##</code><code>##

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

my ($x,$y,$count) = (0,0,0);


for(1..15) {
  
 $count++;
 
 if (fork == 0) {
 
   #$widget->geometry("wxh+x+y")
   my $top = new MainWindow;
   $top->geometry('100x100'.'+'.$x.'+'.$y);
   
   $top->Button(-command => sub { warn "top$_" })->pack;
   MainLoop;
   CORE::exit();
  }

if( $count % 3 == 0) {$y += 150; $x = 0 }
$x += 150;

}
MainLoop;

#If you need start a new process from one of the child processes, then
#you have to establish some kind of IPC (e.g. pipes) between the child
#and parent.

#I the launched task running another Perl/Tk module or function or a
#complete new program? In the latter case, you should just use fork and
#exit/system and do not forget to use CORE::exit instead of exit in the
#child process (according to the FAQ). Perl/Tk forks are somewhat more
#difficult. You have to make sure that you fork off the process which
#does not have a created a MainWindow itself. This will work fine: