What you're looking at is a hash of hashes. Each source IP address is given its own hash of corresponding destinations. $srciphash{$ip} is a reference to that hash.
I'd highly recommend looking at the Data Structures Cookbook and perlref POD pages -- these concepts are explained there better than I could hope to do.
blokhead
Comment on Re: nested hashes and their usage e.g $foo{bar}{baz}