Sounds like a data corruptions to me. It even gets weirder. I added the following to your code:

BEGIN {
    use Carp;
    no strict 'refs';
    *{"bbb::()"} = sub {confess};
}
[download]

That resulted in:

Recursive inheritance detected while looking for method '(abs' in pack
+age 'bbb'.
[download]

Removing the BEGIN block again, and running your program with the -Do flag to perl gives:

(/opt/perl/lib/5.8.6/base.pm:74)        Looking for method isa in pack
+age aaa
(/opt/perl/lib/5.8.6/base.pm:74)        Looking for method isa in pack
+age UNIVERSAL
(/opt/perl/lib/5.8.6/base.pm:74)        Looking for method isa in pack
+age bbb
(/opt/perl/lib/5.8.6/base.pm:74)        Looking for method isa in pack
+age UNIVERSAL

EXECUTING...

(xx:0)  Recalcing overload magic in package bbb
(xx:0)  Looking for method () in package bbb
(xx:0)  Looking for method () in package aaa
(xx:0)  Looking for method () in package bbb
(xx:0)  Looking for method () in package aaa
....
(xx:0)  Looking for method () in package bbb
(xx:0)  Looking for method () in package aaa
Recursive inheritance detected while looking for method '()' in packag
+e 'bbb'.
(xx:0)  Recalcing overload magic in package IO::Handle
(xx:0)  Looking for method () in package IO::Handle
(xx:0)  Looking for method () in package UNIVERSAL
(xx:0)  Checking overloading of `DESTROY' in package `IO::Handle'
(xx:0)  Looking for method DESTROY in package IO::Handle
(xx:0)  Looking for method DESTROY in package UNIVERSAL
(xx:0)  Looking for method AUTOLOAD in package IO::Handle
(xx:0)  Looking for method AUTOLOAD in package UNIVERSAL
[download]