```python
LADDER = (("m", 1000), ("cm", 10), ("mm", 1))


def split_length(mm: int) -> str:
    if not isinstance(mm, int) or mm < 0:
        raise ValueError("Length must be a non-negative integer.")
    
    result = ""
    for unit, multiplier in LADDER:
        count = mm // multiplier
        if count:
            result += f"{count}{unit}"
            mm %= multiplier
    
    return result if result else "0mm"
```