Re^2: A few Perl OOP questions. (disparaging) shows what happens when the common Perl idiom $class = ref $class || $class isn't followed and someone expects it to be implemented that way. I would thus argue that someone expecting new to act like clone when it isn't explicitly documented is relying on implementation details of the class. Even if it is documented, it's still a bad idiom.

If you want alternatives, here they are:

package Foo;
use base 'Clone::PP'; # And don't bother writing it yourself

sub new 
{
    my $class = shift;
    my $self  = { };
    # Intitilze $self
    bless $self, $class;
}
[download]

Although people shouldn't be expecting the constructor to make a clone or copy anyway, this idiom is so common that it may be wise to explicitly document that your constructor won't work that way. Better yet, have new die if it receives a reference.