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