require Carp;

{
   package TestA;
   sub testing { TestC::testing(); }
}

{
   package TestB;
   sub testing { TestD::testing(); }
}

{
   package TestC;
   *testing = \&TestB::testing;
}

{
   package TestD;
   sub testing { print(Carp::confess("Confessing")); }
}

TestA::testing();

__END__
Confessing at !.pl line 20
        TestD::testing() called at !.pl line 10
        TestB::testing() called at !.pl line 5    <-- We called TestC:
+:testing
        TestA::testing() called at !.pl line 23
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