The documentation seems to imply that the variable is tied somehow so changes will be automatically applied, but I don't see code to do that anywhere. It looks like if you replace your line

with

it seems like it does what I think you want.

use warnings;
use strict;

use Tk;
use Tk::Optionbox;

my $newTop = MainWindow->new;

my $vsel;

my @options = ( qw/ This That Amazon Th'other too many to fit on the s
+creen/);

my $selectVendor = $newTop->Optionbox(-text=>'Vendor',
                   -command => \&sayit,
                   -variable=> \$vsel,
                  # -font=>$variableFont,
                   -options=> [@options],
                   -rows =>15,
                  )->pack(-side=>'left');

$vsel = 'Amazon';
$selectVendor->set_option( $vsel, $vsel, $vsel );

MainLoop;

sub sayit { print $_[0], "\n"; }
[download]

Another alternative which seems to work is to change -variable to -textvar:

#!/usr/bin/perl -w

use strict;
use warnings;
use feature qw{ say };
use Tk;
use Tk::Optionbox;

my $default_vsel = "Amazon";

my @options = (
    'option1',
    'option2',
    $default_vsel,
    'option4',
);

my $mw           = Tk::MainWindow->new(-title => 'Tk::Optionbox exampl
+e');
my $vsel         = $default_vsel;
my $variableFont = 'Arial';
my $newTop       = $mw->Frame->pack(-expand => 1, -fill => 'x');

my $selectVendor = $newTop->Optionbox(
                     -command  => \&select_vendor,
                     -textvar  => \$vsel,
                     -font     => $variableFont,
                     -options  => [@options],
                     -rows     => 15,
                   )->pack(-side=>'left');

Tk->MainLoop;


sub select_vendor {
    my $vendor = shift;
    say "Vendor is '$vendor'";
}
[download]

Thanks thundergnat and golux Both solutions work.