The CPAN shell (and module) automates or at least simplifies the building and installation of Perl modules and extensions. It includes some primitive searching capabilities and knows how to use Net::FTP, LWP, and certain external download clients to fetch distributions from the Internet. Then it automatically tests and installs them and their dependencies.
IP::Country is a tool for fast lookup of country codes from IP addresses. Finding out the country of a client using only the IP address can be difficult. Looking up the domain name associated with that address can provide some help, but many IP address are not reverse mapped to any useful domain, and the most common domain (.com) offers no help when looking for country. This module comes bundled with a database of countries where various IP addresses have been assigned. Although the country of assignment will probably be the country associated with a large ISP rather than the client herself, this is probably good enough for most log analysis applications, and under test has proved to be as accurate as reverse-DNS and WHOIS lookup.
Coro is a package providing both low-level coroutines as well as a high-level multitasking abstraction on top of Perl. It allows you to parallelize LWP and other network protocols with ease. Unlike the so-called "Perl threads/fork emulation", they provide a real shared address space (like real threads) without the associated race conditions and performance loss, while maintaining a very low memory overhead (2-3kb/coroutine). The package comes with examples and many supporting modules.
IO::AIO implements fully asynchronous I/O (not non-blocking I/O) that lets your program stay in control and interactive even when doing heavy disk (or other) I/O. It provides asynchronous versions of read(), write(), open(), stat(), unlink(), rename(), utime(), readlink(), and most other standard syscalls as well as interfaces to sendfile(), readahead(), and other esoteric syscalls, complete with emulation on systems that don't support them. It also offers convenience functions to load whole files and more. It fits naturally into any event loop model, supports 9 priorities and request grouping.
JSON::XS implements JSON for Perl. Unlike other modules, its primary goal is to encode to syntactically correct JSON and flag invalid JSON while decoding. It ensures round-trip integrity of data types while being intuitive to use. Currently being the fastest of the JSON encoders available for Perl, it supports a variety of format options, such as single-line, ASCII-only, or pretty-printed, and can be tuned for speed or memory usage. It comes with a wealth of documentation describing usage and implementation details.