$available_data{'02'}{'03'} = {unit => '02', lesson => '03', next => undef};

$available_data{'02'}{'05'} = {unit => '02', lesson => '05', next => undef};
$available_data{'02'}{'03'}{next} = $available_data{'02'}{'05'};

$available_data{'02'}{'11'} = {unit => '02', lesson => '11', next => undef};
$available_data{'02'}{'05'}{next} = $available_data{'02'}{'11'};

etc...

##</code><code>##

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

##</code><code>##

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'};