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

$selectVendor->configure(-variable => \$vsel);

with

$selectVendor->set_option( $vsel, $vsel, $vsel );

it seems like it does what I think you want.

See test code below:

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"; }
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