perlfun_syswrite.txt

    syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
    syswrite FILEHANDLE,SCALAR,LENGTH
    syswrite FILEHANDLE,SCALAR
            Attempts to write LENGTH bytes of data from variable SCALAR to
            the specified FILEHANDLE, using write(2). If LENGTH is not
            specified, writes whole SCALAR. It bypasses buffered IO, so
            mixing this with reads (other than "sysread)"), "print",
            "write", "seek", "tell", or "eof" may cause confusion because
            the perlio and stdio layers usually buffer data. Returns the
            number of bytes actually written, or "undef" if there was an
            error (in this case the errno variable $! is also set). If the
            LENGTH is greater than the data available 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 characters counting backwards from the end of
            the string. If SCALAR is of length zero, you can only use an
            OFFSET of 0.

            WARNING: If the filehandle is marked ":utf8", Unicode characters
            encoded in UTF-8 are written instead of bytes, and the LENGTH,
            OFFSET, and return value of "syswrite" are in (UTF8-encoded
            Unicode) characters. The ":encoding(...)" layer implicitly
            introduces the ":utf8" layer. Alternately, if the handle is not
            marked with an encoding but you attempt to write characters with
            code points over 255, raises an exception. See "binmode",
            "open", and the open pragma.