Should the XS implementation give the (overridable) isa method the precedence?

Only if you want Perl's OO to keep working. In all other cases -- every one -- Perl calls overridden methods which optionally call super methods.

(Modern versions of UNIVERSAL explicitly recommend against calling UNIVERSAL::isa directly.)

So if I understand you correctly, it's recommented to do:

package A;

sub foo {};

package B;

sub bar {};

package C;

use vars qw(@ISA);
@ISA = qw(B);

sub foo {};
sub isa {
  return $_[1] eq 'A' or $_[0]->SUPER::isa(@_);
}
[download]

against:

package A;

sub foo {};

package B;

sub bar {};

package C;

use vars qw(@ISA);
@ISA = qw(A B);
[download]

Or at least it's an allowed and wanted to work way? Further: It's allowed and wanted to work that:

package C;

use vars qw(@ISA);
@ISA = qw(B);

sub foo {};
sub isa {
  return $_[1] eq 'A';
}
[download]

To prevent, C is no B?