```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 whole number and non-negative.")
    
    units = []
    for unit, multiplier in LADDER:
        count = mm // multiplier
        if count > 0:
            units.append(f"{count}{unit}")
            mm %= multiplier
    
    return " ".join(units) if units else "0mm"
```