And I've encountered some features, described in camel book(s), which just don't work in some (or most, or modern ) versions of perl, for example:

001  while ($sth=~/sth(sth)/g)
002   {}

shortcut. This used to work in 5.000, stopped working in 5.6 because of broken optimisation. I reported the bug, but apparently this was considered a buglet, and left as is.

This forced me, (and for example Apache::ASP project) to instead use unefficient replacement:

001  $mycopyofstring=$sth;
002  while ($mycopyofstring=~s/sth(sth)/) {
003  }

I don't know what you're trying to show here, but the second example is a syntaxt error (Substitution replacement not terminated).

The point here, is that important features of language appear and disappear at will, thus you've got to stay in-loop all the time, keeping and eye on changes, hoping that what you've learned won't be thrown out of the window next sumer.

Important features? I think you mean experimental features.