Thanks for posting the details. Expect 1.21 is about ten years older than 1.35 (2007 vs 2017). Since I'm unable to reproduce your issue with Expect 1.35 on both versions of Perl, I am guessing that the issue lies with one of the bugs that was fixed in Expect over those 10 years. I'd say your best course of action is to upgrade the module.

Update: Sorry, I see now that you're getting your expected behavior on the older version of the module instead of the newer version. The Changelog does mention "Eliminate $` and $' from the code. part of (RT #61395) This fix might break some existing code n some extreme cases when the regex being matched has a lookbehind or a lookahead at the edges." which could potentially be a hint, but finding out if this actually is the issue will take a bit more digging.