```python
def pack_entries(entries: list) -> str:
    if not isinstance(entries, list):
        raise ValueError("Input must be a list")
    
    def escape_string(s: str) -> str:
        return s.replace("\\", "\\\\").replace("=", "\\=").replace(";", "\\;")
    
    escaped_entries = [f"{escape_string(key)}={escape_string(value)}" for key, value in entries]
    return ";".join(escaped_entries)
```