Personally I tend to use Data::Dumper. (I prefer setting $Data::Dumper::Indent to 1.) Admittedly that is slow and memory wasteful for data structures with tens of thousands of elements.

Some people prefer the output of YAML. I haven't used it though.

Either will dump a hash of arrays in a readable format, and won't break if your data structure isn't quite as regular as you thought.

Beyond that, if you are going to write your own code for dumping a hash of arrays, I would strongly recommend indenting the array elements some and outdenting the hash keys so that visual scan can tell you where you are in the data structure.

Cheers,
Ben