I was surprised by my test results on Travis-CI, where my module failed on 5.16, 5.18, and 5.20, but not on the versions above or below those.
Doing some digging, it's because of the way that stat works on an in-memory filehandle: on most versions, it gives a warning in the unopened subcategory of the io warnings category, so I set up my module to trap on that warning, and die with a more meaningful message to the end-user, which should help them avoid the problem. But when it ran on multiple versions on Travis-CI, I found that 5.16-5.20 apparently do not give that (or any) warning, but still result in the empty list for the in-memory filehandle.
The issue can be simply replicated with the one-liner SSCCE (shown in both windows/berrybrew and linux/perlbrew) in the spoiler below.
I found a workaround (see second spoiler below), but I was curious, so I skimmed the perldelta for 5.16 and 5.22, but couldn't find a mention of the unopened-filehandle warning changing in either of those versions. So I guess the wisdom I am seeking: does anyone have insight as to why the warning disappeared in 5.16, but came back in 5.22? Was it intentional (and if so, why try removing it, and why re-instate it)? Or was it an accidental bug (if so, why wasn't it mentioned, or did I just not notice)?
And thank you: while working on the earlier issues -- figuring out that in-memory filehandle's don't stat right, but give the misleading "unopened filehandle" warning, even though Scalar::Util::openhandle() shows it as open; and trying to get the __WARN__ handler correct -- I thought I was going to need to SoPW, but developing a SSCCE showed me the problem. So thanks for the Rubber duck that you didn't even know you provided, too. :-)