XML::Parser versions through 2.47 for Perl has an off-by-one heap buffer overflow in st_serial_stack.
In the case (stackptr == stacksize - 1), the stack will NOT be expanded. Then the new value will be written at location (++stackptr), which equals stacksize and therefore falls just outside the allocated buffer.
The bug can be observed when parsing an XML file with very deep element nesting
Apply the patch that has been publicly available since 2006-06-13.
Apply the patch that has been publicly available since 2006-06-13 or upgrade to version 2.48 or later when it is released.