Thanks for the reply. In the production code I do use strict & warnings. Should have in the test program too. Oops. AH! Excellent! I am clearing the whole hash. Duh! Thanks so much. And thanks for the tip. Big thanks!! ;-) For completeness, the new code works.

use Tk;
use strict;
use warnings;

require Tk::Entry;
require Tk::Button;

my $mw = MainWindow->new();

my %work = (
    "VM-1" => "da923",
    "VM-2" => "dz427");

main_display(\%work);

MainLoop;

# -----------------------------------------------------
sub main_display {
    my $ref_hash = shift(@_);

    my $lab1 = $mw->Label(-textvariable => \$ref_hash->{'VM-1'},
                                            -relief => 'sunken',
                                            -width => 10,
                                            )->pack();
    my $lab2 = $mw->Label(-textvariable => \$ref_hash->{'VM-2'},
                                            -relief => 'sunken',
                                            -width => 10,
                                                )->pack();
    my $but = $mw->Button(-text=>'clear', -command=> [sub{
                                          $ref_hash->{'VM-1'} = " ";
                                          $ref_hash->{'VM-2'} = " ";
                                          $mw -> update;
                                           }] )->pack();

}
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