Close. Your traditional flock will either be blocking or non-blocking. If you only set the LOCK_EX flag, it will block by default, therefore when you try to get the lock it will wait indefinitely.

If this is not what you want, then you should set the LOCK_NB flag as well, and poll at a specified interval a specified number of times before giving up. (You can set an alarm, but Perl's signal handling is not very good so I would not recommend doing that.)

The portable way to get those constants is to

use Fcntl qw(:flock);
# time passes
foreach (1..10) {
  $is_locked = flock(FH, LOCK_EX | LOCK_NB);
  last if $is_locked;           ### Or gets both flags
  sleep 1;
}
# Test $is_locked here
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See the longer example for the full semantics of how to do the open, etc. (There I am locking a sentinel...)

UPDATE
In the above FH is a filehandle that has already been opened. As for the other question, yeah that module will solve the problem. But everything that I said about locking will also hold when you want to start playing games with reading and writing other kinds of data files as well...

UPDATE 2
OK, if you want to do this explicitly for a sentinel, here is some untested code:

use Carp;
use Fcntl qw(:flock);

sub open_sentinel {
  my $lock_file = shift;
  my $fh = do {local *FOO}; # Old trick
  open ($fh, "> $lock_file") or die "Cannot write $lock_file: $!";
  my $is_locked;
  foreach (1..10) {
    $is_locked = flock($fh, LOCK_EX | LOCK_NB);
    last if $is_locked;
    select (undef, undef, undef, 0.01);
  }
  return $is_locked ? $fh : undef;
}
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This will wipe out the file (hence it is only useful to use on a sentinel file). To get the lock just try to grab. The variable will only be defined if you got the lock, and the lock is freed when the variable is destroyed. (So you can enter a routine, ask for a lock, if you get it then proceed, if you don't then exit immediately.)

The above example tries 10 times, one hundredth of a second apart.