Perl5 リファレンス

               Sets FILEHANDLE's system position using the system
               call lseek(2).  It bypasses stdio, so mixing this
               with reads (other than `sysread'), `print',
               `write', `seek', `tell', or `eof' may cause
               confusion.  FILEHANDLE may be an expression whose
               value gives the name of the filehandle.  The
               values for WHENCE are `0' to set the new position
               to POSITION, `1' to set the it to the current
               position plus POSITION, and `2' to set it to EOF
               plus POSITION (typically negative).  For WHENCE,
               you may also use the constants `SEEK_SET',
               `SEEK_CUR', and `SEEK_END' (start of the file,
               current position, end of the file) from the Fcntl

               Returns the new position, or the undefined value
               on failure.  A position of zero is returned as the
               string `"0 but true"'; thus `sysseek' returns true
               on success and false on failure, yet you can still
               easily determine the new position.