# Acquire lock
sysopen(LOCKFILE,'L',O_WRONLY|O_CREAT);
while(!flock(LOCKFILE, LOCK_EX|LOCK_NB)) {
        sleep(...);
}
# Lock granted!
if(I am process A) {
  open(my $file,'<','F');
  ...
} elsif(I am process B) {
  unlink 'F';
  open(my $file,'>','F);
  ...
}
# Release lock
flock(LOCKFILE, LOCK_UN);
close(LOCKFILE);