Well in addition to what my_nihilist said, about reusing toplevel windows, you can extend that to reuse all widgets. Don't try to create/destroy widgets....at best you will get a memory gain for each cycle. Withdraw or packForget them, reconfigure them, then re-pack them.

As far as why the 2 toplevels get the same information, you probably have the module setup wrong and you are not returning a blessed object ( usually called $self in the module). Here is the basics of a Tk module/package

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

package ZCanTree;

#without Tk::Derived the option -dooda will fail
use base qw(Tk::Derived Tk::Canvas);
Tk::Widget->Construct('ZCanTree');

sub ClassInit {
    my ($class, $mw) = @_;
    #set the $mw as parent
    $class->SUPER::ClassInit($mw);
} # end ClassInit


sub Populate {
    my ( $self, $args ) = @_;
#-------------------------------------------------------------------
#take care of args which don't belong to the SUPER, see Tk::Derived
my $xtra_arg = delete $args->{-dooda};  #delete and read same time 
if( defined $xtra_arg ) { $self->{'dooda'} = $xtra_arg }

#-----------------------------------------------------------------
    $self->SUPER::Populate($args);

    $self->{'can'} = $self->Canvas(
             )->pack(-expand=>1,-fill=>'both');

    $self->Advertise( Canvas => $self->{'can'} );

print "2\n";
}

sub get_dooda{
    my $self  = shift;
    return $self->{'dooda'};
}
1;


package main;
use Tk;

my $mw = MainWindow->new;
$mw->geometry("400x400");


my $canf = $mw->ZCanTree( -bg => 'black', 
                          -dooda => 42,
                  )->pack(-fill=>'both',-expand=>1);


my $button = $mw->Button( -text=>'Dooda', 
          -command=> sub{ print $canf->get_dooda(),"\n" } )->pack();

my $button1 = $mw->Button( -text=>'Exit', -command=> sub{exit} )->pack
+();

Tk::MainLoop;
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