Sets FILEHANDLE's system position using the system call lseek(2). It bypasses stdio, so mixing this with reads (other than
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_END (start of the file, current position, end of the file) from the Fcntl module.
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.