Find all pairs of positive integers $m$, $n$ such that the $(m+n)$-digit number \[\underbrace{33\ldots3}_{m}\underbrace{66\ldots 6}_{n}\] is a perfect square.
To find all pairs of positive integers \( m \) and \( n \) such that the \((m+n)\)-digit number \(\underbrace{33\ldots3}_{m}\underbrace{66\ldots 6}_{n}\) is a perfect square, we will analyze the structure and properties of the number.

1. **Express the number in a more manageable form:**
   The number \(\underbrace{33\ldots3}_{m}\underbrace{66\ldots 6}_{n}\) can be written as:
   \[
   N = 3 \cdot \underbrace{111\ldots1}_{m} \cdot 10^n + 6 \cdot \underbrace{111\ldots1}_{n}
   \]
   This is because each digit '3' and '6' can be represented as \(3 \cdot 10^k\) and \(6 \cdot 10^k\) respectively.

2. **Check divisibility by 3:**
   Since the number is composed of digits 3 and 6, it is clearly divisible by 3. We can simplify the problem by dividing the number by 3:
   \[
   \frac{N}{3} = \underbrace{11\ldots1}_{m}\underbrace{22\ldots 2}_{n}
   \]
   Let \( M = \underbrace{11\ldots1}_{m}\underbrace{22\ldots 2}_{n} \).

3. **Analyze the number modulo 4:**
   - If \( n \geq 2 \), then the last two digits of \( M \) are 22, and:
     \[
     22 \equiv 2 \pmod{4}
     \]
     Since 2 is not a quadratic residue modulo 4, \( M \) cannot be a perfect square.

4. **Consider the case \( n = 1 \):**
   - If \( n = 1 \), the number \( M \) ends in 2, and:
     \[
     \underbrace{11\ldots1}_{m}2
     \]
     We need to check if this can be a perfect square. The last digit of a perfect square can only be 0, 1, 4, 5, 6, or 9. Since the last digit here is 2, it cannot be a perfect square.

5. **Consider the case \( n = 0 \):**
   - If \( n = 0 \), the number \( M \) is simply \(\underbrace{11\ldots1}_{m}\), which ends in 1. We need to check if this can be a perfect square. The last digit of a perfect square can be 1, so we need to check if:
     \[
     \underbrace{11\ldots1}_{m}
     \]
     is a perfect square. However, for \( m \geq 2 \), the number \(\underbrace{11\ldots1}_{m}\) is not a perfect square.

6. **Check the smallest possible values:**
   - For \( m = 1 \) and \( n = 1 \), the number is 36, which is a perfect square (\(6^2 = 36\)).

Thus, the only possible ordered pair is \((1, 1)\).

The final answer is \( \boxed{ (1, 1) } \).