Found something interesting while testing this morning. Here's a sample wrapper for this widget:

#! Perl script
use strict;
use warnings;
use Tk;
use SR::Dtest;

my $main = MainWindow->new;
$main->Dtest;
#$main->pack();

MainLoop;
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When invoked this way, I get the unreferenced value error. However, if I do the following:

$main->Dtest(-target=>5);
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... I get the desired result (a label with the string "var=5"). According to Tk::configSpecs, the format is:

configSpecs(-option => [where,dbName,dbClass,default])
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Why isn't ConfigSpecs assigning a default value of 4 to -target?