It is surprisingly obnoxious to fetch the value passed in to perl's -F command line parameter. Icky. Also possible is just reading from the interpreter variable PL_minus_f but that takes XS. The following is the plain perl version.
use B 'main_root'; use List::Util 'reduce'; sub _minus_f { my $minus_f; eval { my $op = reduce { $a->$b } main_root, split ' ', <<'OPTREE'; first sibling sibling first sibling first first sibling first sibling first first sibling first OPTREE $minus_f = $op->precomp; }; return $minus_f; }