One problem with %HAS is that you have to duplicate the list of attributes.

# type them once...
my %field1;
my %field2;
my %HAS;
# type them twice...
{$HAS{$_}=1} foreach qw(field1 field2);
[download]

As soon as you start typing field1, field2, etc. more than once you give yourself a problem when you change one and forget to change the other, etc.

For example, a typo in %HAS will foul up your new() method without any warnings, etc... and we suddenly start having some of the effects that we switched to inside-out-objects to avoid ;-)

There's also the fact that you don't want the user to fiddle with every attribute in your object.

This leads to the more general point that I don't think that a totally generic object construction subroutine is practical. What ever method you pick is going to be inappropriate/annoy one or more groups of people.

So - don't sweat it! Do something that works for the code that you're writing. Don't worry too much about the platonic one-true-new. There ain't no such beast :-)