I'm partial to const subs and namespace purity:

package My::Class;
use vars qw( $VERSION );
BEGIN { $VERSION= 1.01 }

package My::Class::_implement;

BEGIN {
    my $offset= 0;
    for my $member (  qw( FOO BAR BAZ BIF )  ) {
        eval "sub $member() { $offset }; 1"
            or  die "eval failed: $@";
        $offset++;
    }
}

sub My::Class::GetObjectPackage {
    my $class= shift(@_);
    return $class . "::Object";
}

sub My::Class::new {
    my $class= shift(@_);
    bless [], $class->GetObjectPackage();
}

sub My::Class::Object::Foo {
    my $self= shift(@_);
    my $value= $self->[FOO];
    $self->[FOO]= shift(@_)   if  @_;
    return $value;
}
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