diff --git a/crates/perry-codegen/src/expr/array_push.rs b/crates/perry-codegen/src/expr/array_push.rs
index c7410a166c..dfc76038e3 100644
--- a/crates/perry-codegen/src/expr/array_push.rs
+++ b/crates/perry-codegen/src/expr/array_push.rs
@@ -381,13 +381,22 @@ pub(crate) fn lower(ctx: &mut FnCtx<'_>, expr: &Expr) -> Result<String> {
                     );
                     let box_ptr = blk.bitcast_double_to_i64(&cap_dbl);
                     blk.call_void("js_box_set", &[(I64, &box_ptr), (DOUBLE, &new_box)]);
+                    return Ok(emit_array_handle_length(ctx, &new_handle));
                 } else if let Some(slot) = ctx.locals.get(array_id).cloned() {
                     let blk = ctx.block();
                     let box_dbl = blk.load(DOUBLE, &slot);
                     let box_ptr = blk.bitcast_double_to_i64(&box_dbl);
                     blk.call_void("js_box_set", &[(I64, &box_ptr), (DOUBLE, &new_box)]);
+                    return Ok(emit_array_handle_length(ctx, &new_handle));
                 }
-                return Ok(emit_array_handle_length(ctx, &new_handle));
+                // #5459: `array_id` is in `boxed_vars` but has no box location in
+                // THIS context — it's a module-level global accessed directly from
+                // a nested function (the load path read `@global`, not a box-get).
+                // Returning here would skip the realloc write-back entirely, so the
+                // relocated array header is never stored to the registered GC-root
+                // global slot: the old head is freed on the next GC and the global
+                // dangles (use-after-free / corrupted length). Fall through to the
+                // module-global store-back below instead of returning.
             }
             if let Some(&capture_idx) = ctx.closure_captures.get(array_id) {
                 let closure_ptr = ctx
@@ -446,13 +455,18 @@ pub(crate) fn lower(ctx: &mut FnCtx<'_>, expr: &Expr) -> Result<String> {
                     );
                     let box_ptr = blk.bitcast_double_to_i64(&cap_dbl);
                     blk.call_void("js_box_set", &[(I64, &box_ptr), (DOUBLE, &new_box)]);
+                    return Ok(emit_array_handle_length(ctx, &new_handle));
                 } else if let Some(slot) = ctx.locals.get(array_id).cloned() {
                     let blk = ctx.block();
                     let box_dbl = blk.load(DOUBLE, &slot);
                     let box_ptr = blk.bitcast_double_to_i64(&box_dbl);
                     blk.call_void("js_box_set", &[(I64, &box_ptr), (DOUBLE, &new_box)]);
+                    return Ok(emit_array_handle_length(ctx, &new_handle));
                 }
-                return Ok(emit_array_handle_length(ctx, &new_handle));
+                // #5459: in `boxed_vars` but no box location here — a module-level
+                // global accessed directly from a nested function. Fall through to
+                // the module-global store-back so the relocated head reaches the
+                // GC-root slot (see the matching note in `Expr::ArrayPush`).
             }
             if let Some(&capture_idx) = ctx.closure_captures.get(array_id) {
                 let closure_ptr = ctx.current_closure_ptr.clone().ok_or_else(|| {
diff --git a/crates/perry/tests/issue_5459_module_global_array_push_in_closure.rs b/crates/perry/tests/issue_5459_module_global_array_push_in_closure.rs
new file mode 100644
index 0000000000..213d93adbb
--- /dev/null
+++ b/crates/perry/tests/issue_5459_module_global_array_push_in_closure.rs
@@ -0,0 +1,120 @@
+//! Regression test for #5459 — use-after-free when `arr.push(...)` targets a
+//! module-level array from inside a nested function (closure/IIFE).
+//!
+//! Root cause: in `lower/expr/array_push.rs`, the `boxed_vars` write-back branch
+//! returned early after handling the captured/local-box cases. A module-level
+//! global that is in `boxed_vars` (because a nested function references it) but
+//! has NO box location in the callee context falls through both inner arms — so
+//! the early return skipped the realloc write-back entirely. When `js_array_push_f64`
+//! relocated the array head, the new head was never stored back to the
+//! GC-root global slot: the old head was freed on the next GC and the global
+//! dangled (garbage `.length`, freed elements → SIGSEGV under churn).
+//!
+//! The fix only returns early when a box location was actually written;
+//! otherwise it falls through to the module-global store-back
+//! (`emit_root_nanbox_store_on_block`). Same fix for `ArrayPushSpread`.
+
+use std::path::PathBuf;
+use std::process::Command;
+
+fn perry_bin() -> PathBuf {
+    PathBuf::from(env!("CARGO_BIN_EXE_perry"))
+}
+
+fn compile_and_run(dir: &std::path::Path, entry: &std::path::Path) -> (bool, String) {
+    let output = dir.join("main_bin");
+    let compile = Command::new(perry_bin())
+        .current_dir(dir)
+        .arg("compile")
+        .arg(entry)
+        .arg("-o")
+        .arg(&output)
+        .output()
+        .expect("run perry compile");
+    assert!(
+        compile.status.success(),
+        "perry compile failed\nstdout:\n{}\nstderr:\n{}",
+        String::from_utf8_lossy(&compile.stdout),
+        String::from_utf8_lossy(&compile.stderr)
+    );
+    let run = Command::new(&output).output().expect("run compiled binary");
+    (
+        run.status.success(),
+        String::from_utf8_lossy(&run.stdout).to_string(),
+    )
+}
+
+/// A module-level array populated from inside an IIFE must survive allocation
+/// churn + gc() with all elements intact (pre-fix: SIGSEGV / garbage length).
+#[test]
+fn module_global_array_pushed_in_iife_survives_gc() {
+    let dir = tempfile::tempdir().expect("tempdir");
+    let entry = dir.path().join("main.ts");
+    std::fs::write(
+        &entry,
+        r#"
+declare function gc(): void;
+function churn(n: number): void {
+  let j: any[] = [];
+  for (let i = 0; i < n; i++) { j.push({ i, p: "z".repeat(40) + i }); if (j.length > 128) j = []; }
+}
+const strong: any[] = [];
+(function setup() { for (let n = 0; n < 50; n++) strong.push({ id: n }); })();
+for (let c = 0; c < 12; c++) { churn(80000); gc(); }
+if ((strong.length as number) !== 50) { console.log("BADLEN " + strong.length); }
+let alive = 0;
+for (let n = 0; n < 50; n++) if (strong[n] && strong[n].id === n) alive++;
+console.log("survived " + alive);
+"#,
+    )
+    .expect("write entry");
+
+    let (ok, stdout) = compile_and_run(dir.path(), &entry);
+    assert!(
+        ok,
+        "binary crashed (use-after-free regression)\nstdout:\n{stdout}"
+    );
+    assert!(
+        stdout.contains("survived 50"),
+        "all 50 module-global array elements must survive GC (got: {stdout})"
+    );
+    assert!(
+        !stdout.contains("BADLEN"),
+        "module-global array length must stay 50 across GC (got: {stdout})"
+    );
+}
+
+/// Same hazard via spread-push (`arr.push(...xs)`) into a module-global from a
+/// nested function.
+#[test]
+fn module_global_array_spread_pushed_in_iife_survives_gc() {
+    let dir = tempfile::tempdir().expect("tempdir");
+    let entry = dir.path().join("main.ts");
+    std::fs::write(
+        &entry,
+        r#"
+declare function gc(): void;
+function churn(n: number): void {
+  let j: any[] = [];
+  for (let i = 0; i < n; i++) { j.push({ i, p: "z".repeat(40) + i }); if (j.length > 128) j = []; }
+}
+const acc: any[] = [];
+(function setup() { for (let n = 0; n < 25; n++) acc.push(...[{ id: n }, { id: n + 100 }]); })();
+for (let c = 0; c < 12; c++) { churn(80000); gc(); }
+let ok = 0;
+for (let n = 0; n < acc.length; n++) if (acc[n] && typeof acc[n].id === "number") ok++;
+console.log("spread " + ok + "/" + acc.length);
+"#,
+    )
+    .expect("write entry");
+
+    let (ok, stdout) = compile_and_run(dir.path(), &entry);
+    assert!(
+        ok,
+        "binary crashed (spread use-after-free regression)\nstdout:\n{stdout}"
+    );
+    assert!(
+        stdout.contains("spread 50/50"),
+        "all 50 spread-pushed elements must survive GC (got: {stdout})"
+    );
+}