There probably is a way to dig into the Menubutton structure to remove the added menubuttons, but it probably would be easier to somehow packForget the menu, then rebuild it. Otherwise, it's probably better to use something that allows the -options to be an array reference. So all you need do to change it, it to change arrays.

#!/usr/bin/perl
use warnings;
use strict;
use Tk;

my $mw = MainWindow->new();

my @opt1 = ('a'..'m');
my $var = 'a';
my $tvar = 'a';

my $opt = $mw->Optionmenu(
                          -command => \&show_choice,
                          -variable => \$var,
                          -textvariable => \$tvar,
                          -options => \@opt1,
                         )->pack;


$mw->Button(-text=>'Change Options',-command=>[\&change_ops, $opt])->p
+ack;

$mw->Button(-text=>'Exit', -command=>sub{$mw->destroy})->pack;

MainLoop;

sub show_choice
{
    print "got: ", shift, "\n"
}

sub change_ops
{
    my $op = shift;
    my @newoptions =();
    $tvar = 'null';
    $op->configure( -options => \@newoptions );
}
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