if (-e "$file") {
     open WRITER, "+<$file", $file or die "Can't open: $!";
     flock WRITER, LOCK_EX or die "Can't lock $file: $!";
     truncate WRITER, 0 or die "Can't truncate $file: $!";
} else {
     open WRITER, ">$file" or die "Can't open: $!";
     flock(WRITER, LOCK_EX or die "Can't lock $file: $!";
}

#do whatever....