error when exit codes returns 0

[J-E-N]

hope someone could help me out.

when the exit code returns 0, the page displays "Internal Server Error. The server 
encountered an internal error or misconfiguration and was unable to complete your 
request." but the fetchmail was done successfully.

$fetch = system ("/usr/bin/fetchmail -aK -f/tmp/$uid.rc");
 if ($fetch == 0) { 
    print "successful";
 } else {
    print "Error: $?"; 
 }

thanks :)