I saw the same error message when I create menu with $mainwindow->Menu, cascade, and command. When I create menu with Menubutton and change label with entryconfigure, label changed successfully.

use strict;
use warnings;

use Tk;
my ($mw);#,$menubar,$file,@lables, @label_texts);
$mw=MainWindow->new;

my $menub=$mw->Menubutton(-text => "File",
    -menuitems => [
        [ 'command' => "New", "-command" => \&newfile, "-underline" =>
+ 0 ],
        [ 'command' => "Open", "-command" => \&openfile, "-underline" 
+=> 0 ],
        [ 'command' => "Save", "-command" => \&savefile, "-underline" 
+=> 0 ],
        [ 'command' => "SaveAs", "-command" => \&saveasfile, "-underli
+ne" => 4 ] ] )->pack

$mw->Button(-text=>'test', -command=>\&test)->pack;

MainLoop;

sub test{
    print "in test\n";
    foreach my $i (1 .. 4){
        $menub->entryconfigure($i,-label=>'8 million ways to die (1986
+)[HD].TAG' );
    }
}
[download]

regards.