DB<132> sub cycle { $_[1]++; $_[1] %= $_[0] }

  DB<133> cycle 3 => $h{a}{b}[2]{c};   \%h
 => { a => { b => [undef, undef, { c => 1 }] } }

  DB<134> cycle 3 => $h{a}{b}[2]{c};   \%h
 => { a => { b => [undef, undef, { c => 2 }] } }

  DB<135> cycle 3 => $h{a}{b}[2]{c};   \%h
 => { a => { b => [undef, undef, { c => 0 }] } }