I cannot speak for that hypothetical "we", but I as a module developer support my users because I want my code to be useful and used. This means that I don't want to place an undue version burden on my users, especially when I am one of my module users. I cannot easily upgrade the Perl versions I have available, and hence want my code to be both, conveniently maintainable on one side and conveniently working on the other side.

For me, this means that I use whatever features I see fit - for example I often use the three-argument open, but I never use our, because the benefit of our is not existent. It's not "big extra work" for me because I mostly know what breaks between the major versions of Perl, and that "little extra work" usually pays off when I encounter a machine with only 5.005 on it.