Good day.
Some time ago we discuss that a quantified assertions can be used:
https://lists.exim.org/lurker/message/20110502.103121.96e51b9d.en.html
Here is pcretest listing:
PCRE2 version 10.31 2018-02-12
/(?<=(?=.)?)/
Failed: error 125 at offset 0: lookbehind assertion is not fixed length
ab
Assertion have a fixed 0 length not matter it quantified or not.
So such patterns shouldn't rise error.
It seems it's no error in Perl.