Would your Holiness care to explain what is wrong with undef in the role of undetermined predecessor?
Update: People appear to not like the tone of this question.
No offence is intended. 'your Holiness' seemed like a nice and proper way to address a perl saint.
Another saint (demerphq) and others have discussed the problems around $a++ and $a-- (not) being eachothers inverse. Thank you for that.