# factor out accumulating behavior:

    sub make_acc_fn(&@) {
        my $f = shift;
        $f->(@_);
        $f;
    }

    # mean

    sub mean { mean_acc(@_)->() }   # all-at-once version
    sub mean_acc {                  # accumulating-function version
        my ($tot, $cnt);
        make_acc_fn {
            $cnt += @_;
            $tot += $_ for @_;
            $cnt ? $tot / $cnt : undef;
        } @_ ;
    }

    # uniq

    sub uniq { uniq_acc(@_)->() }
    sub uniq_acc {
        my %uniq;
        make_acc_fn {
            @uniq{ @_ } = ();
            wantarray ? keys %uniq : scalar keys %uniq;
        } @_ ;
    }


    # examples

    print mean(1,2,3,5), $/;
    # 2.5

    my $m = mean_acc();
    print "$_ => ", $m->($_), $/ for 1..5;
    # 1 => 1
    # 2 => 1.5
    # 3 => 2
    # 4 => 2.5
    # 5 => 3

    print uniq(1,2,3,1..5), $/;
    # 41325

    my $u = uniq_acc(1,2,3);
    print "$_ => ", $u->($_), $/ for 1..5;
    # 1 => 132
    # 2 => 132
    # 3 => 132
    # 4 => 4132
    # 5 => 41325