According to the Perl 5.8 delta,

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 differently. 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. The restricted hashes interface is expected to be the replacement interface (see Hash::Util). If your existing programs depends on the underlying implementation, consider using Class::PseudoHash from CPAN.