```python
def split_length(mm: int) -> str:
    if not isinstance(mm, int) or mm < 0:
        raise ValueError("Invalid length")
    
    ladder = [("m", 1000), ("cm", 10), ("mm", 1)]
    result = []
    
    for unit, factor in ladder:
        count = mm // factor
        if count > 0:
            result.append(f"{count}{unit}")
        mm %= factor
    
    return " ".join(result) or "0mm"
```