Are you checking the output of your commands?
What changes during that day that causes a command to fail at one time, but work at other times?
Update to clarify:
From system:

The return value is the exit status of the program as returned by the wait call. To get the actual exit value divide by 256. See also exec. This is not what you want to use to capture the output from a command, for that you should use merely backticks or qx//, as described in `STRING` in the perlop manpage. Return value of -1 indicates a failure to start the program (inspect $! for the reason).