Perl5 リファレンス

                   lock I

               This function places an advisory lock on a
               variable, subroutine, or referenced object
               contained in THING until the lock goes out of
               scope.  This is a built-in function only if your
               version of Perl was built with threading enabled,
               and if you've said `use Threads'.  Otherwise a
               user-defined function by this name will be called.
               See the Thread manpage.