my %solutionMatrix;	# hash of lists that contain platforms and customer variants per platform
my $defaultPlatform = "Mod1";	# the default platform, selected at program start
$solutionMatrix{"Mod1"} = [ 'a','b','c' ];	# store the list address in the hash for default key
$solutionMatrix{"Mod2"} = [ 'q' ]; # another platform

	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 to default value