You have bound probably not the exact thing you wanted. This binds button release for button one (left button) and two (middle button). Try

$list_box->bind('<Button-1>',
                sub {
                  my $indx = $list_box->curselection();
                  my $selected_object = $list_box->get($indx);
                });
$list_box->bind('<Double-Button-1>', 
                sub{
                  print “\nuser clicked twice”;
                });
[download]

All of this good event binder information is in perldoc Tk::bind.