Like sgifford, I believe the problem lies in writedata or release_lock (or in some other code you've not shown us).
I also believe your code should be cleaned up a bit, such as:
use Fcntl qw':flock :seek';
our $COUNT_FILE = 'mycounter';
? open $cfh, "+< $COUNT_FILE"
: open $cfh, "+> $COUNT_FILE" # not needed if file always exists
or die "Can't open $COUNT_FILE: $!\n";
flock($cfh, LOCK_EX) or die "Can't lock: $!\n";
chomp(my $count = <$cfh> || '');
seek($cfh, 0, SEEK_SET);
print $cfh ++$count, "\n";
print "Run #$count\n";
# truncate not needed as $count is always increasing
close($cfh); # automagically releases the lock
Black flowers blossom
Fearless on my breath