diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 88a56448..07296ad2 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -94,6 +94,7 @@
   "pancake_sort.ml": 910,
   "pascal.ml": 252,
   "peano.ml": 30,
+  "perfect.ml": 3,
   "pi_leibniz.ml": 314,
   "prefix_sum.ml": 66,
   "pretty_table.ml": 64,
diff --git a/lib/ocaml/baseline/perfect.ml b/lib/ocaml/baseline/perfect.ml
new file mode 100644
index 00000000..2ec3c4a9
--- /dev/null
+++ b/lib/ocaml/baseline/perfect.ml
@@ -0,0 +1,23 @@
+let div_sum n =
+  let s = ref 1 in
+  let i = ref 2 in
+  while !i * !i <= n do
+    if n mod !i = 0 then begin
+      s := !s + !i;
+      let q = n / !i in
+      if q <> !i then s := !s + q
+    end;
+    i := !i + 1
+  done;
+  if n = 1 then 0 else !s
+
+let count_perfect limit =
+  let c = ref 0 in
+  for n = 2 to limit do
+    if div_sum n = n then c := !c + 1
+  done;
+  !c
+
+;;
+
+count_perfect 500
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index 34acfbec..4de83aaf 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,12 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-10 Phase 5.1 — perfect.ml baseline (count perfect numbers
+  ≤ 500 = 3). Perfect numbers = those where d(n) = n. Three under
+  500: 6, 28, 496. (8128 is the next.) Same div_sum machinery as
+  euler21_small / abundant. Original 10000 limit timed out under
+  contention; 500 stays under budget. 125 baseline programs total —
+  milestone.
 - 2026-05-10 Phase 5.1 — abundant.ml baseline (count abundant numbers
   < 100 = 21). Abundant means d(n) > n where d(n) is the proper-
   divisor sum. Reuses the trial-division div_sum helper from iter