how rename() actually works on read-only files just went over my head.

rename doesn't modify any files; it modifies a directory or two. As long as you have write access to the source and destination directories, you can rename the file.

$ touch foo

$ chmod 0 foo

$ ls -li
total 0
11314128152 ---------- 1 ikegami pg1404028 0 Dec 11 10:11 foo

$ perl -e'rename("foo", "bar") or die $!'

$ ls -li
total 0
11314128152 ---------- 1 ikegami pg1404028 0 Dec 11 10:11 bar

$ chmod a-w .

$ perl -e'rename("bar", "baz") or die $!'
Permission denied at -e line 1.

$ ls -li
total 0
11314128152 ---------- 1 ikegami pg1404028 0 Dec 11 10:11 bar
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