# With selftest declared after _self
% perl -MO=Deparse test.pl
...
sub Data::Validate::OO::selftest (@) {
    package Data::Validate::OO;
    my $self = &_self(\@_);                 # <---- look here
    print Dumper(@_);
}
...

# Now with selftest declared after _self
% perl -MO=Deparse test.pl
...
sub Data::Validate::OO::selftest (@) {
    package Data::Validate::OO;
    my $self = &_self(@_);                  # <---- and here
    print Dumper(@_);
}
...