Perl5 リファレンス



       syswrite FILEHANDLE,SCALAR
               Attempts to write LENGTH bytes of data from
               variable SCALAR to the specified FILEHANDLE, using
               the system call write(2).  If LENGTH is not
               specified, writes whole SCALAR.  It bypasses
               stdio, so mixing this with reads (other than
               `sysread())', `print', `write', `seek', `tell', or
               `eof' may cause confusion because stdio usually
               buffers data.  Returns the number of bytes
               actually written, or `undef' if there was an
               error.  If the LENGTH is greater than the
               available data in the SCALAR after the OFFSET,
               only as much data as is available will be written.

               An OFFSET may be specified to write the data from
               some part of the string other than the beginning.
               A negative OFFSET specifies writing that many
               bytes counting backwards from the end of the
               string.  In the case the SCALAR is empty you can
               use OFFSET but only zero offset.