Well, there-in lies the solution. Just setting the attribute of -textvariable a second time fixed the issue.

Adding 

$platformDropdown -> configure( -textvariable => \$platformChoice );

[download]
to the end does the trick.... which pokes the TK engine and says "Hey, set the value to THIS". Even though the variable already contained the right value, I guess TK needs to have some sort of event to know it needs to update. So the corrected code, that now works, looks like: 
my %solutionMatrix;    
my $defaultPlatform = "Mod1";
$solutionMatrix{"Mod1"} = [ 'a','b','c' ];
$solutionMatrix{"Mod2"} = [ 'q' ]; 

    my $platformDropdown = $selectionFrame -> Optionmenu ( -textvariab
+le => \$platformChoice, -indicatoron => 1 ) -> pack( -side => "left")
+;

        # --- now populate the options in the platform and customer dr
+opdown list,  
        #  which is in the hash of arrays, %solutionMatrix
        foreach my $platform ( sort keys %solutionMatrix ) {
            # $platform is a choice for the platform dropdown
            # @{$solutionMatrix{$platform} is an array of solutions fo
+r the customer dropdown
            $platformDropdown -> addOptions([$platform=>$platform]);
        }

        $platformChoice = $defaultPlatform;    # set default
        $platformDropdown -> configure( -textvariable => \$platformCho
+ice );

[download]

In reply to Re^2: Tk Optionmenu - how to set the default selection by Minok
in thread Tk Optionmenu - how to set the default selection by Minok

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