configure.pl should update the value of $useFHS when it does the installation. Did you install from Github using makeDist then
configure.pl? Or did you manually copy the files?
I noticed in github, backuppc3 lib/BackupPC/Lib.pm has $useFHS = 1, while master (v4) has $useFHS = 0. While that shouldn't matter (since
configure.pl updates it), I will change master to $useFHS = 1.