USN-222-1 fixed a vulnerability in the Perl interpreter. It was discovered that the version of USN-222-1 was not sufficient to handle all possible cases of malformed input that could lead to arbitrary code execution, so another update is necessary.
Jack Louis of Dyad Security discovered that Perl did not sufficiently check the explicit length argument in format strings. Specially crafted format strings with overly large length arguments led to a crash of the Perl interpreter or even to execution of arbitrary attacker-defined code with the privileges of the user running the Perl program.
However, this attack was only possible in insecure Perl programs which use variables with user-defined values in string interpolations without checking their validity.