```python
def fill_template(template: str, values: dict) -> str:
    if not isinstance(template, str):
        raise ValueError("Template must be a string")
    
    result = []
    i = 0
    while i < len(template):
        if template[i] == '{':
            j = i + 1
            while j < len(template) and template[j].isalnum() or template[j] == '_':
                j += 1
            if j == i + 1 or template[j] != '}':
                raise ValueError("Unterminated placeholder")
            name = template[i+1:j]
            if not name:
                raise ValueError("Empty placeholder name")
            if name in values:
                result.append(values[name])
            else:
                raise ValueError(f"Unknown placeholder: {name}")
            i = j + 1
        else:
            result.append(template[i])
            i += 1
    
    return ''.join(result)
```