After installing GNU libidn 1.35 as follows

/usr/local/lib/libidn.12.dylib
/usr/local/lib/libidn.a
/usr/local/lib/libidn.dylib -> libidn.12.dylib
/usr/local/lib/libidn.la

/usr/local/include/idn-free.h
/usr/local/include/idn-int.h
/usr/local/include/idna.h
/usr/local/include/pr29.h
/usr/local/include/punycode.h
/usr/local/include/stringprep.h
/usr/local/include/tld.h
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I was able to build Net::LibIDN using

Note: I installed GNU libidn 1.35 (the version just prior to GNU libidn2) alongside GNU libidn2.