See the current Perl documentation for sysread.
Here is our local, out-dated (pre-5.6) version:
sysread - fixed-length unbuffered input from a filehandle
Attempts to read
LENGTH bytes of data into variable
SCALAR from the specified
FILEHANDLE, using the system call
read(2). It bypasses stdio, so mixing this with other kinds of reads,
seek(), or tell() can cause confusion because stdio usually buffers data. Returns the number
of bytes actually read,
0 at end of file, or undef if there was an error.
SCALAR will be grown or shrunk so that the last byte
actually read is the last byte of the scalar after the read.
OFFSET may be specified to place the read data at some place in the string other than the beginning.
OFFSET specifies placement at that many bytes counting backwards from the end of the string.
OFFSET greater than the length of
SCALAR results in the string being padded to the required size with
"\0" bytes before the result of the read is appended.