use Fcntl ':flock';
sub alter_file {
  open(HANDLE1, ...);
  flock(HANDLE1, LOCK_EX)  # exclusive lock
    or die "unable to obtain lock: $!";
  chop; chop; chop;
  close(HANDLE); # also releases lock
}

sub process {
  open(HANDLE2, ...);
  flock(HANDLE2, LOCK_SH)    # shared lock - allows multiple readers
    or die "unable to obtain shared lock: $!";
  ...read from file...
  close(HANDLE2);  # also releases lock
}

##</code><code>##

sub alter_file {
  open(HANDLE1, ">/some/tmp/file");
  ...write to HANDLE1...
  close(HANDLE1);
  rename("/some/tmp/file", "file");
}