I didn't see anyone mention this. 5.8.0 had a bug under some OSes where setting $0 just didn't work. Linux was one of these affected. This was fixed in some later release, but I don't know which 5.8.x that was.

I got trapped by this and other bits, needing to wait for a post-5.8.0 release, and then getting hit with "but now the system is too important to risk upgrading." But somehow arranging for the backup second system hasn't been important ...