use strict;
use warnings;
use Fcntl qw(LOCK_EX LOCK_NB);

open DATA, ">>", $0 or die $!;

flock DATA, LOCK_EX | LOCK_NB or die "already running\n";

print "started $$\n";
sleep 300;
__DATA__

##</code><code>##

$ ./761299.pl
started 1220690
^Z
Suspended
$ bg
[1]    ./761299.pl &
$ ./761299.pl
already running
$ kill 1220690
[1]    Terminated                    ./761299.pl
$ ./761299.pl
started 1220692

##</code><code>##

kfcntl(3, F_SETLK, 0x2FF22430)   = 0              # when not already running

kfcntl(3, F_SETLK, 0x2FF22430)   Err#13 EACCES    # otherwise