Pseudohash behavior will change. Quoth 'perldoc perlref' in a recent development version of perl:

NOTE: The current user-visible implementation of pseudo- hashes (the weird use of the first array element) is deprecated starting from Perl 5.8.0 and will be removed in Perl 5.10.0, and the feature will be implemented differ ently. Not only is the current interface rather ugly, but the current implementation slows down normal array and hash use quite noticeably. The 'fields' pragma interface will remain available.

So, if you want to use pseudohashes do it through fields::phash. As for why to use a pseudohash, what the fields pragma does is a good example. You can prevent yourself from accidentally accessing the "wrong" hash entry most of the time (and thus all the difficult to track down bugs that can cause).