I see what you mean, but personally I would see errors in Perl on such basic and well-used build-ins would be unlikely. If it was a subtle timing issue then it is possible that a shell would appear to solve it because it would probably take longer. mkdir(1) is not a shell built-in normally (it might be on recent versions of ksh), so that would invoke two child processes just to run one kernel API call. A bit of an over-kill.