Modules (usually XS, but some pragmata are pure Perl) that are intricately tied to the core and are effectively "optional builtins".

You mean the modules called "core extensions" in Config::Extensions? 

perl -MConfig::Extensions=%Extensions -e 'for (sort{lc$a cmp lc$b}keys
+ %Extensions){print"$_\t$Extensions{$_}\n"}'

[download]
Why are some core modules referred to as "extensions"?

In reply to Re^2: What modules should be added to the corelist? by Anonymous Monk
in thread What modules should be added to the corelist? by Anonymous Monk

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