You can use Parallel::ForkManager or Proc::Queue for that:

# untested
use Proc::Queue size => 4, ignore_children => 1, qw(run_back);

sub docalc {
    run_back {
        my $filename = $_;
        my $filenamewithpath = $File::Find::name;
        my $directoryname = $File::Find::dir;
        my $workdirectory = ...;
        mkpath($workdirectory);
        copyfiles(...);
        chdir $workingdirectory;
        open STDOUT, '>', "$workbasedirectory/$filename.log";
        do "/.../scriptthatincludesfortran.pl"
    } or warn "unable to fork new process to handle '$_': $!"
}
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