```python
def fill_template(template: str, values: dict) -> str:
    if not isinstance(template, str):
        raise ValueError("fill_template expects a string template")
    
    result = ""
    placeholder_index = 0
    
    for char in template:
        if char == "{":
            placeholder_name = template[placeholder_index + 1:placeholder_index + len(values)]
            placeholder_index += len(values) + 2
            
            if not placeholder_name.isalnum() or any(c not in "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_" for c in placeholder_name):
                raise ValueError(f"fill_template: invalid placeholder name '{placeholder_name}'")
            
            if placeholder_name not in values:
                raise ValueError(f"fill_template: unknown placeholder '{placeholder_name}'")
            
            result += str(values[placeholder_name])
        else:
            result += char
    
    return result
```