Returns the current position in bytes for FILEHANDLE, or -1 on error. FILEHANDLE may be an expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted, assumes the file last read.
Note the in bytes: even if the filehandle has been set to operate on characters (for example by using the :encoding(utf8) open layer), tell will return byte offsets, not character offsets (because that would render seek and tell rather slow).
The return value of tell for the standard streams like the STDIN depends on the operating system: it may return -1 or something else. tell on pipes, fifos, and sockets usually returns -1.
There is no systell function. Use sysseek($fh, 0, 1) for that.
Do not use tell (or other buffered I/O operations) on a filehandle that has been manipulated by sysread, syswrite, or sysseek. Those functions ignore the buffering, while tell does not.