from typing import Dict, Any

def python_dict_first_value_len(d: Dict[Any, Any]) -> int:
    r"""
    Computes the length of the first value in a dictionary.

    - If the first value is a `list`, returns its length.
    - If the first value is a `dict`, returns the number of keys in the dictionary.
    - Otherwise, returns `1` for scalar values.

    Args:
        d (Dict[Any, Any]): A dictionary where the first value's length will be computed.

    Returns:
        int: The computed length based on the type of the first value.

    Raises:
        ValueError: If `d` is `None` or empty.
    """
    if not d:
        raise ValueError("Input dictionary is None or empty.")

    first_value = next(iter(d.values()))  # Get the first value

    if isinstance(first_value, list):
        return len(first_value)
    elif isinstance(first_value, dict):
        return len(first_value.keys())  # Return the number of keys in the dictionary
    else:
        return 1  # Scalar case (int, float, str, etc.)