This was asked recently by somebody, in a slightly different form, on how to remove the indicator. Some responded with this. Maybe you can figure out how to change it, rather than undef it.? If you do post it.

As a last resort, you can take the code from the Optionmenu module, which is derived from MenuButton, and include them as packages into your code, and customize them.

#!/usr/bin/perl
use Tk;

my $mw = MainWindow->new();

my $opt = $mw->Optionmenu(
        -options => [[jan=>1], [feb=>2], [mar=>3]]
               )->pack;
#$opt->configure(-indicatoron => undef); 

my $button = $mw->Button(
        -text => 'Remove Indicatoron',
        -command => sub{ $opt->configure(-indicatoron => undef);   }
)->pack;

MainLoop;
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