Item Description: the standard module for peeking at Perl's internals
If you've ever wanted to know just how Perl is storing your data internally, this module is for you. Its most important function is Dump($sv), which prints a full description of the internal state of $sv: flags, contents, type, recerence count, etc. If $sv is a reverence, Dump() will also print the referred-to object. The output is much more technical that that of Data::Dumper, but if you suspect a bug in how Perl is handling your data, this is one of the best tools for checking. Simply Dump() your data structure and see if everything is there that should be.
This module also provides a set of other, much more deeply embued-with-black-magic functions, such as ones which will retrieve memory usage statistics if your Perl was built using Perl's own malloc() and ones which let the programmer directuly manipulate a variable's reference count from withing Perl. This is not something that any normal Perl programmer should ever need (or want) to do, but if you're looking to learn more about the guts of the Perl 5 interpreter, this module is a great place to start.