You are using the four-argument version of read:

read FILEHANDLE,SCALAR,LENGTH,OFFSET

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.

This would to me explain the behaviour you're seeing, as Anonymous Monk already diagnosed as well.