perlbase-unicore perhaps? (from https://openwrt.org/packages/index/languages---perl) which is detailed in the Embedded Perl: Installing Perl and Mojolicious on Arduino Yśn / OpenWRT ChaosCalmer I linked to earlier, where the author faced a similar problem "I got gcc and gnu make working on OpenWRT but EXTERN.h is missing from the Perl core and I'm trying to figure out why."..."I got it working!!!", and a link to the tutorial they made.

Update: OP states they tried to install this (Re^2: EXTERN.h missing when compiling on OpenWrt), but "(the dependencies are not properly reflected in the packages)". Perhaps something to do with the OS version being out of support.

Thanks for all your effort.

As for the tutorial you mention, it seems that it is not quite complete and some things seem to be just a little bit different on my system.

E.g. I do not need to build make, but I need to install perlbase-dumpvalue to make cpanm work.

But I can use cpanm like this now:

curl -L https://cpanmin.us | perl - -M https://cpan.org -n <module>
[download]

Again I need to use cpan.org and not cpan.metacpan.org as in the tutorial, as otherwise the modules are not found.

But of course when I try this to install Net::SSLeay it fails becuase of the missing EXTERN.h.

I is totally unclear to me how the author of this tutorial who initially also had the problem of a misssing EXTERN.h managed to make this problem go away.

This is magic that I cannot reproduce.