1. Introspection

Does Perl provide any way to programmatically list the "names" of all the Perl built-in functions? By "name" I mean the string, e.g. "y", that when appended to the string "CORE::" can be given to CORE::prototype as its argument to obtain the prototype of the corresponding Perl built-in.

2. Overriding built-ins

Some Perl built-ins are overridable, others aren't. What is the best way to find out which are the ones that cannot be overridden?

For all non-overridable built-ins CORE::foo, the expression CORE::prototype('CORE::foo') returns undef, but the converse is not true. For example, CORE::prototype('CORE::system') returns undef, but CORE::system is overridable, as shown by the following snippet:

use strict;
use warnings;

use subs 'system';
sub system { warn "this is not the real system\n" }

warn "about to test system\n";
system 'date';
warn "done testing system\n";
__END__
about to test system
this is not the real system
done testing system
[download]

For other built-ins with a CORE::prototype return value of undef, however, the above trick won't work. Either the overriding function will be silently ignored (as happens if one tries to override CORE::print), or Perl will react more spectacularly, such as seg-faulting if one tries to override CORE::require.

So, is there a way to determine which built-ins are overridable, other than trial and error?

(Both questions prompted only by curiosity.)