package Void;
sub void_sub { 'void sub' }
package Empty;
sub empty_sub { 'empty sub' }
package Nothing;

@Nothing::ISA = qw( Void Empty );

sub isa { 0 }

my $uni_isa;
BEGIN { $uni_isa = \&UNIVERSAL::isa; }
{   
    no warnings 'redefine';  # Subroutine UNIVERSAL::isa redefined

    sub UNIVERSAL::isa {
        ref $_[0] eq __PACKAGE__ ? 0 : goto &$uni_isa;
    }
}
package main;

my @refs = ( ref {}, ref [], ref \do{my $x}, ref sub {}, ref qr// );
my $nothing = bless {}, 'Nothing';
foreach my $reftype ( @refs ) {
    ok( ! UNIVERSAL::isa( $nothing, $reftype ),
        "\$nothing is not $reftype" );
    ok( ! $nothing->isa( $reftype ), "! \$nothing->isa( $reftype )" );
}
ok( UNIVERSAL::isa( [], 'ARRAY' ), 'isa still works' );
ok( ! UNIVERSAL::isa( {}, 'ARRAY' ), 'isa still works (negation)' );
is( $nothing->void_sub(), 'void sub', 'method dispatch still works' );