I'd give each window a "title", then iconify.

#!/usr/bin/perl

use warnings;
use Tk;

my $mw = MainWindow->new();
my $mw2 = MainWindow->new();
my $mw3 = MainWindow->new();

$mw-> title ('Win1');
$mw2-> title ('Win2');
$mw3-> title ('Win3');

my $c = $mw->Canvas(-width => 200, -height => 200);
my $c2 = $mw2->Canvas(-width => 200, -height => 200);
my $c3 = $mw3->Canvas(-width => 200, -height => 200);
$c->pack;

MainLoop;
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