I didn't say auto-vivification was the problem. I said that using undefined references was his problem. Auto-vivification is saving his butt. He should be using a hash here. Trying to emulate a hash by using a sparse matrix is difficult, error prone and wrong.

In general the code is very poor; the OP uses map when foreach would do, he uses foreach when map or grep is called for, he iterates over an array to create a hash when he could have hashed the data to begin with, he has multiple arrays holding the same data, he uses push when a hash is called for and he uses indexing where a push would serve. Arguing over auto-vivification is akin to rearranging the deck chairs while the ship is sinking.