It's not at all hard to see it – an example is not necessary. The perldoc quoted in the first reply says everything: the trick works by means of join(), such that it would have been far better if the feature had not been a part of the language and programmers had to type a few extra letters into their 300-baud teletypes. If your data contains the separator-character, the code breaks. Whereas, if you (today) use hash-of-hashes and auto-vivification, you can quickly construct real multi-dimensional data structures without concern for what the hash-key values might be, and the code is no more cumbersome. Some of the "older" ideas in Perl were really good ones. But this is not one of them.