Document the fix for perl 5.10 with this comment:

Document the fix for perl 5.10 with this comment:
 * The temporary enabling of the caller opcode here is to work around a
 * bug in perl 5.10, which unkindly changed the way its Safe.pm works, without
 * notice. It is quite safe, as caller is informational only, and in any case
 * we only enable it while we load the 'strict' module.