You can use a reference as a hash key, it just is not useful to do so. The reference will be stringified, and cannot be converted back to the data it originally referred to. The Perl References manual page covers this towards the end, under the appropriate heading of "WARNING".

The exact same reference will always stringify the same way, and in that sense it could be used with effect as a hash key. However, you would still have to have another means by which to map the stringified key back to the original ref. The section above gives one example of how to do this.

--rjray