I don't think it is documented, and it is due to trying to grandfather something in.

POSIX.pm has an import method which is a wrapper around Exporter which is meant to allow things matching /^\w+_h$/ to have ":" appended making them tags. If you look at Exporter's documentation, tags for groups only work if the first element is a tag. So in your first example the second element is turned into a tag, and Exporter barfs on it not being exported. In your second example all works.

For a hack to fix it, find POSIX.pm, search for "import", and add a sort of @list before it calls Exporter's import function.