#!/usr/bin/perl
use strict;
use warnings;
use Fcntl qw/:DEFAULT :flock/;

print "$$: Before open.\n";
sysopen (FILE, 'lockedfile', O_RDWR | O_CREAT) or die "$$: cannot open file: $!\n";

### Einfacher, legt die Datei aber nicht an, wenn sie noch nicht existiert:
# open FILE, '+<', 'lockedfile' or die "cannot open file (proc $$): $!\n";

print "$$: Before flock.\n";
flock FILE, LOCK_EX;
print "$$: Writing to file.\n";
print FILE "Here is process $$.\n";
print "$$: Sleeping.\n";
sleep 30;
print "$$: Closing file.\n";
close FILE;
print "$$: Finished.\n";