libsrs2 is the next generation SRS library. SPF verifies that the Sender address of an email message matches (according to some policy) the client IP address that submitted it. When a message is forwarded, the sender address must be rewritten to comply with SPF policy. The Sender Rewriting Scheme, or SRS, provides a standard for this rewriting that is not vulnerable to attacks by spammers, is easy to parse by common mail filters, and handles multiple hops neatly and safely.
qpsmtpd is a flexible smtpd daemon written in Perl. Apart from the core SMTP features, all functionality is implemented in small "extension plugins" using the easy-to-use object oriented plugin API. qpsmtpd was originally written as a drop-in qmail-smtpd replacement, but now it also includes an smtp forward and a postfix "backend".
Apache SpamAssassin is an extensible email filter that is used to identify spam. Once identified, the mail can then be optionally tagged as spam for later filtering. It provides a command line tool to perform filtering, a client-server system to filter large volumes of mail, and Mail::SpamAssassin, a set of Perl modules allowing Apache SpamAssassin to be used in a wide variety of email systems.
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.
CGI::ContactForm is a Perl module that generates a contact form for the Web and sends a message to a predestinated recipient. Unlike many form-to-mail programs, it's limited to this single purpose, but it allows you to set up a decently designed form with a minimum of effort. In other words, it's simple to use rather than flexible, even if there are quite a few customization possibilities.
Net::SMPP is a Perl module that implements versions 3.4, 4.0, and 5.0 of the SMPP protocol for ESME and SMSC roles. SMPP is commonly used by clients and servers for sending SMS messages in mobile network. This module allows you to implement an SMS client, an SMS server, or even an SMS proxy. It supports all standard TLVs and some vendor specific TLVs as well.
VoiceXML::Client is a library that provides Perl programs with the ability to act as a user agent that fetches Voice Extensible Markup Language (VoiceXML) documents, parses them, and then executes the instructions therein by playing prompts and collecting user DTMF input/voice recordings through a handle to some type of telephone or other device interface. VoiceXML::Client's focus is on the client side. The XML documents are generated by a distinct process of your choice, unrelated to the library.