Recently, looking at Class::Std, a friend of mine complained that it was failing its tests on 5.6.1; it complained about deep recursion in AUTOLOAD. He later said that the problem was this:
# this line starts in column 0, and seems like runtime assignment
*ID = \&Scalar::Util::refaddr;
# the same for these, later:
*_extract_default = _extractor_for_pair_named('default');
*_extract_init_arg = _extractor_for_pair_named('init_arg');
*_extract_get = _extractor_for_pair_named('get');
*_extract_set = _extractor_for_pair_named('set');
# later, in Class::Std::SCR...
my %values_of : ATTR ( :init_arg<values> );
The handler for ATTR ends up getting called at compile time, and it, in turn, uses _extract_default. That isn't defined yet (because the ATTR handler (MODIFY_HASH_ATTRIBUTES) is being called at compile time) so AUTOLOAD gets called. AUTOLOAD, in turn, uses ID, which isn't defined yet, so it calls AUTOLOAD...!
Deep recursion happens, and we all cry.
Why isn't this a problem under, say, perl 5.8.7?