WWW::RobotRules was added for 5.8.0. It was only added because it was part of the libwww distribution which was added because it was a prerequisite for some other module that Jarkko wanted to add (see how complex it can get). Math::Complex has been in the core for years. If nothing else it serves as a good example of operator overloading.

As for adding modules to the core, I think we need a good date/time module in there. I suggest DateTime and its friends.

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg

libwww is included because CPAN uses it. CPAN belongs in core because you use it to install whatever else you need. Thus, anything CPAN uses belongs in core too. This makes sense, if you think about it: whatever else you want (whether it's XML::Simple or Acme::Bleach) you can easily install _if_ your perl came with what is needed to fetch and install it from CPAN.

This assumes that your distro will come with whatever is needed to build XS (a C compiler, make, and so forth), but Perl comes from a Unix background where that is a mostly reasonable assumption.

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Quidquid latine dictum sit altum viditur.