It is too early when you call the values by curselection (before MainLoop, i.e. before the user had a chance to interact with the window). Try something like

my $mw=new MainWindow;
$lb = $mw->Listbox( -selectmode =>"multiple")->pack;
$lb->insert('end', @dir);
$mw->Button(-text => "ok", -command=> \&dosomething)->pack;

MainLoop;
# ----------------------------
sub dosomething {
  my @selected = $lb->curselection;
  print scalar @selected;
  $mw->destroy();
} # dosomething
[download]

(code not tested)

due to the callback, &dosomething is called when the button is pressed, i.e. while MainLoop is running