Here's an accessor/mutator that is set up for chaining and will accept undef as a value.

sub foo {
    my $self = shift;

    # Set attribute if any args
    if ( @_ )  {
        $self->{foo} = shift;  #validation is for chumps

        return $self;
    }
    else {
        return $self->{foo}
    }

    die "Whiskey Tango Foxtrot";
}

# cotrast with this version, which I think you had in mind
sub undefoo {
    my $self = shift;
    my $newfoo = shift;

    if ( defined $newfoo ) {
        $self->{foo} = $newfoo;  # What, me validate input?
        return $self;
    } 
    else {
       return $self->{foo};
    }

    die "Whiskey Tango Foxtrot";
}
[download]

print $obj->foo(undef), "\n"; # Prints object stringification
print $obj->undefoo(undef), "\n"; # prints a "\n"
[download]

I don't know how I feel about chaining mutators. I like accessor/mutators that always return the attribute value.