package YourPackage;
use strict;use warnings;
use Tk;
use Tk::Canvas;
sub new {
   my $class = shift;
   my $self = bless {}, $class;
   my $mw = MainWindow->new();
   $self->{'mainwin'}=$mw;
   $self->{'mycanvas'}=$mw->Canvas();
   return $self;
}
sub getcanvas {
   my $self=shift;
   printf "Returning a %s\n",ref($self->{'mycanvas'});
   return $self->{'mycanvas'}
}
package main;
my $YP = YourPackage->new();
my $canvas=$YP->getcanvas();