In IkiWiki/Wrapper.pm, the gen_wrapper function finds out what srcdir and destdir are set to in the config, but does not use them.
Later in the sub, when a new wiki.cgi wrapper is being created when calling ikiwiki --setup /path/to/setup, it will only work if cgi_wrapper in the config file is set to the full path. Otherwise, it creates wiki.cgi in the current working directory. It works with the other wrapper it sets up in my config - post_update (using git), as that shows in the config with a full path.
One workaround would be to mention in the setup file that cgi_wrapper has to be the full path, not just the file name, but that seems silly when destdir is also specified in that file and that's where it should go, and $config{destdir} is a known value in the Wrapper.pm file.
Nowhere in any documentation does it say that cgi_wrapper is relative to the destdir. As noted in discussion, there are web server setups that require the cgi be located elsewhere. done --Joey
A comment in the generated setup file that all paths should be full would prevent my (admittedly dumb) error without any drawbacks.