my $mw = MainWindow->new;
my $lb1 = $mw->Scrolled(Listbox);
my $lb2 = $mw->Scrolled(Listbox);
my $b = $mw->Button(-text=>"doit", -command=>sub{&do_it});
MainLoop;

sub do_it {
my $i1 = $lb1->index('active');
my $i2 = $lb2->index('active');
# $i1 and $i2 are always inside 0..end
...
}