A zero and empty string are (mostly) the same thing as far as Perl is concerned. The only time I worry about ditching undef values is if my interface exlicitly states that undef is different from any other false value. Otherwise you're asking to get a lot of 'use of unintitilized value' warnings.

BTW--I like your @$self{keys %SELF) = @{+shift}{keys %SELF}; example, but it deserves some comments for the poor maintance programmer, who may not have as much Perl-foo as you do.