Why is PL/Python untrusted?

According to the docs: > PL/Python is only available as an "untrusted" language, meaning it does not offer any way of restricting what users can do in it and is therefore named plpythonu. A trusted variant plpython might become available in the future if a secure execution mechanism is developed in Python. Why exactly is it difficult to develop a secure execution mechanism for Python but not for other languages such as Perl?