use strict;
use warnings;

$| = 1;

sub A::foo {print "This is A::foo\n"}
@B::ISA = qw /A/;

print "Calling B->foo(): "; B->foo();
print "Calling B::foo(): "; B::foo();

__END__
Calling B->foo(): This is A::foo
Calling B::foo(): Undefined subroutine &B::foo called at ...