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.

In reply to Re^9: EXTERN.h missing when compiling on OpenWrt by marto
in thread EXTERN.h missing when compiling on OpenWrt by morgon

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