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$available_data{'02'}{'03'} = {unit => '02', lesson => '03', next => u
+ndef};

...
$available_data{'02'}{'05'}{next} = $available_data{'02'}{'11'};

etc...

$ref = $available_data{$unit}{$lesson};
print "Unit: $unit, lesson: $lesson -> Unit: ", $ref->{unit}, ' lesson
+: ', $ref->{lesson};

my @available_data;
push @available_data, {unit => '02', lesson => '03'};
push @available_data, {unit => '02', lesson => '05'};
push @available_data, {unit => '02', lesson => '11'};
push @available_data, {unit => '03', lesson => '02'};