From my limited understanding, the method cache allows Perl to remember to which function a method call resolves so it doesn't have to traverse @ISA every time.

Changing the symbol table clears the cache. the value of *foo changes twice in the snippet in your post, the explicit assignment and the implicit assignment at the end of local's scope. 

$o->method();  # Normal (first call)
$o->method();  # Faster due to cache
*foo = sub {};
$o->method();  # Normal (cache cleared)
$o->method();  # Faster due to cache

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In reply to Re^3: private recursive subroutines by ikegami
in thread private recursive subroutines by Sixtease

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