$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');
  }
}

XS_Tk__Callback_Call error:Can't call method "insert" on an undefined value at /etc/perl/chefkoch/list.pm line 30.

Tk::Error: Can't set -options to `ARRAY(0x9d8bd88)' for Tk::Optionmenu=HASH(0x9dcffe0): Can't call method "insert" on an undefined value at /etc/perl/chefkoch/list.pm line 30.
 at /usr/lib/perl5/Tk/Derived.pm line 294

 at /usr/lib/perl5/Tk/Derived.pm line 306
 Tk callback for .frame2
 Tk::Derived::configure at /usr/lib/perl5/Tk/Derived.pm line 306
 Tk::Widget::new at /usr/lib/perl5/Tk/Widget.pm line 205
 Tk::Widget::__ANON__ at /usr/lib/perl5/Tk/Widget.pm line 256
 main::list_1 at /etc/perl/chefkoch/list.pm line 15
 main::list at ck1.3_main.plx line 61
 Tk callback for .frame1.button1
 Tk::__ANON__ at /usr/lib/perl5/Tk.pm line 250
 Tk::Button::butUp at /usr/lib/perl5/Tk/Button.pm line 175
 <ButtonRelease-1>
 (command bound to event)