I couldn't remember the name of Module::CoreList, the module that tells you in which version modules in core made it into the core. I thought it was CoreVersion, but searching for core and coreversion and version on cpan didn't turn it up.
[Actually going back, now I find it's between 300-400 of the 1456 modules CPAN search returns for 'core'. But that's too far at the back. I didn't find it, anyway. I was turned off by Lingua::Stem, README.hpux, Chemistry::Atom in the list before it. All good modules, probably, but not at this moment.]
Googling on 'perl how tell what version core module dual life cpan made' also didn't turn it up.
I hope this message will make it easier for me when I'm looking for it next time. Easier because either Google finds it, or I remember I posted here about it.
I asked on freenode's #perl and someone gave me the answer straight away. Buubot there also answers questions of the form 'core:Module::CoreList'
You can use Module::CoreList this way to find when it made its own way into the core.
perl -MModule::CoreList -e "print Module::CoreList->first_release('Module::CoreList')"
Why do I want to know? I want to know how much I may require a user to install by using certain modules.
But perhaps there is a more extensive way to answer such questions.