Perl5 リファレンス

               Implements the fcntl(2) function.  You'll probably
               have to say

                   use Fcntl;

               first to get the correct constant definitions.
               Argument processing and value return works just
               like `ioctl' below.  For example:

                   use Fcntl;
                   fcntl($filehandle, F_GETFL, $packed_return_buffer)
                       or die "can't fcntl F_GETFL: $!";

               You don't have to check for `defined' on the
               return from `fnctl'.  Like `ioctl', it maps a `0'
               return from the system call into `"0 but true"' in
               Perl.  This string is true in boolean context and
               `0' in numeric context.  It is also exempt from
               the normal -w warnings on improper numeric

               Note that `fcntl' will produce a fatal error if
               used on a machine that doesn't implement fcntl(2).
               See the Fcntl module or your fcntl(2) manpage to
               learn what functions are available on your system.