DB<160> $cycle = sub { $_[0]++; $_[0] %= $_[1] }
 => sub { "???" }

  DB<161> $h{a}{b}[2]{c}->$cycle(3);   \%h
 => { a => { b => [undef, undef, { c => 1 }] } }

  DB<162> $h{a}{b}[2]{c}->$cycle(3);   \%h
 => { a => { b => [undef, undef, { c => 2 }] } }

  DB<163> $h{a}{b}[2]{c}->$cycle(3);   \%h
 => { a => { b => [undef, undef, { c => 0 }] } }