$ perl -e'
   open(my $fh, "<", "/etc/passwd") or die $!;

   my $fileno = fileno($fh);
   defined($fileno) or die "Not a file handle";
   $fileno >= 0 or die "Not system file handle";
   my $qfn = readlink("/proc/$$/fd/$fileno") or die $!;

   CORE::say $qfn;
'
/etc/passwd