my %solutionMatrix;    
my $defaultPlatform = "Mod1";
$solutionMatrix{"Mod1"} = [ 'a','b','c' ];
$solutionMatrix{"Mod2"} = [ 'q' ]; 

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

        # --- now populate the options in the platform and customer dropdown 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 for the customer dropdown
            $platformDropdown -> addOptions([$platform=>$platform]);
        }

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