my $unit;
	my $lesson;

	$available_data{'02'}{'03'} = undef;
	$available_data{'02'}{'05'} = undef;
	$available_data{'02'}{'11'} = undef;
	
	$available_data{'05'}{'02'} = undef;
	$available_data{'05'}{'03'} = undef;
	
	$available_data{'07'}{'01'} = undef;
	$available_data{'07'}{'04'} = undef;
	$available_data{'07'}{'07'} = undef;
	$available_data{'07'}{'08'} = undef;	

	foreach $unit (sort keys %available_data)
	{
		print "Unit " . $unit . " consists of lessons: ";
		foreach $lesson (sort keys %{$available_data{$unit}})
		{
			print $lesson . " ";
		}
		print "\n";
	}

##</code><code>##

Unit 02 consists of lessons: 03 05 11 
Unit 05 consists of lessons: 02 03 
Unit 07 consists of lessons: 01 04 07 08