$listbox = $mw -> Listbox (-selectmode => "single") -> pack();
$optionmenu = $mw -> Optionmenu (-options => [qw/foo bar bla blubb/]
                                 -variable => \$var,
                                 -command => \&subfunktion
                                ) -> pack();

sub subfunktion{
  opendir (FOO, "/usr/share/$var") || die "öffnen: $!";
  @foo = readdir FOO;
  closedir FOO || die "schließen: $!";
  foreach $bar (@foo){
    $listbox -> insert ('end', '$bar');
  }
}