Yes, saying "//=" is concise and more accurate in many instances, but does it really provide any real advantage over using "||=" in the majority of use cases?
Given that it's more accurate, yes. Accuracy is a fine feature in many programs—perhaps even most. I like getting the right answers and I like having fewer bugs than more bugs.
Besides, it's a feature of a Perl release that's nearly six years old. If you want it on a Perl that's even further out of date, you can easily get patches for Perl 5.8 and perhaps even 5.6... but if that's just too much work for you, it's trivial to replace instances of it with a combination of defined and multiple appearances of the relevant variable.
Worse still, IMO, is using Moose (and MooseX) when it is trivial to use traditional OO Perl to do the same thing.
I just can't see it as a terrible thing that people who volunteer to give away their code for other people to use freely choose to do things that make their volunteer work enjoyable. You might as well suggest that we stop using Perl and start using the combination of shell, awk, sed, C, and other Unix tools because that's compatible with SysV in its purest form circa 1986. (Imagine how awful it would be if POSIX features were available to people who didn't know how to use them in C, let alone implement them.)
... but this is all a silly argument for you to make, because you've already poisoned the well by claiming that all features in Perl newer than your personal cutoff point are merely "cool" and not useful.