Yeah, I was a bit concerned that a hash of hashes of arrays was a bit complex in this case. Sometimes I find it easier to think about the levels backward. There's an array of the lines kept in 'line', and a reference to each 'line' is kept in its own $match hash. A reference to each $match is kept in %connection to hold it all together. The 'count' is just another branch of that tree. Set $DEBUG to 1 and look at the data structure.

I've found some quotes about data structures I'd like to share before I start giving bibliography.

• "Bad programmers worry about the code. Good programmers worry about data structures and their relationships." -- Linus Torvalds
• Show me your flowcharts and conceal your tables, and I shall continue to be mystified. Show me your tables, and I won't usually need your flowcharts; they'll be obvious. -- Fred Brooks.
• Data dominates. If you've chosen the right data structures and organized things well, the algorithms will almost always be self-evident. Data structures, not algorithms, are central to programming. -- Fred Brooks
• "It is better to have 100 functions operate on one data structure than to have 10 functions operate on 10 data structures." —Alan J. Perlis

If you don't know who those people are or why I've chosen them to quote, then I suggest a bit of research on them. Their writing will make you a better programmer. As will stuff by Rob Pike, Al Aho, and many others for that matter.

Besides the wonderful Modern Perl already mentioned in the thread, there are other resources, too.

• Chapter 9 of Programming Perl is all about data structures as they apply to Perl.
• perldsc is too.
• The data structures portion of Tom Christiansen (tchrist)'s Far More Than You Ever Wanted To Know series.