I am not building a threaded perl. Here is the relevant snippet from the perlbrew log:
Getting the current patchlevel... (You have perl5 version 24 subversion 4.)
Perl can be built to offer a form of threading support on some systems To do so, Configure can be run with -Dusethreads.
Note that Perl built with threading support runs slightly slower and uses slightly more memory than plain Perl.
If this doesn't make any sense to you, just accept the default 'n'. Build a threading Perl? n
But there are sections of the build log that state the configure script is finding -lpthread and such....
I am running macOS High Sierra 10.13.4, which I think is the latest.