[Please trim your quotes!]
But in any ahem Unix system, you can already do,
ad hoc:
$ ls | perl -n -e 'chomp;
m/(.*)\.log$/&& rename($_, "$1.old");'
I think using perl hardly counts as "any [] Unix system".
Or is that what your "ahem" is there for?
Also, that's going to misbehave, one way or another, on names
containing newlines, and possibly (I'm not entirely clear what "chomp"
does) names containing other characters.
I have a lot of files containing newlines, so yes, this is a serious
problem. What does DOS do with files containing newlines...