|laziness, impatience, and hubris|
I think if you try to submit a bug report for a module, saying that it works wrong when you redefine several perl special variables, nobody will agree to fix that.
But why? It's not wrong to assign a value to those special variables. That's what they are for. If you don't ever assign values to them, there's no reason they'd need to exist at all.
So, why would somebody reject such a bug report? Maybe because it's unsual that it's done in code that isn't a oneliner.
But bug with filename '0' is different case. It's a bug, absolutely.
But one can argue that using 0 as a file name is also unusual.
What do you think about modules that can't handle file names that contain quotes? Or a newline? Is that also a bug, absolutely? What about not handling non-ASCII file names (for which there seems to be no cross-platform way of handling them corectly)?
You draw a line and say "not handling non-default $, is not a bug", but you say "not handling 0 filenames is a bug". Based on what criteria do you draw that line? And where do you draw it?
For the record, I too would consider not handling a filename of 0 a bug, but I lack your perspective on which corner cases need to be handled, and which are OK to ignore.