Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE, using read(2). It bypasses buffered IO, so mixing this with other kinds of reads, print, write, seek, tell, or eof can cause confusion because the perlio or stdio layers usually buffer data. Returns the number of bytes actually read, 0 at end of file, or undef if there was an error (in the latter case $! is also set). SCALAR will be grown or shrunk so that the last byte actually read is the last byte of the scalar after the read.
An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many characters counting backwards from the end of the string. A positive 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.
There is no syseof() function, which is ok, since eof doesn't work well on device files (like ttys) anyway. Use sysread and check for a return value for 0 to decide whether you're done.
Note that if the filehandle has been marked as :utf8, Unicode characters are read instead of bytes (the LENGTH, OFFSET, and the return value of sysread are in Unicode characters). The :encoding(...) layer implicitly introduces the :utf8 layer. See binmode, open, and the open pragma.