use strict;
use Fcntl qw(F_SETLK F_WRLCK SEEK_SET);

BEGIN {
        # c2ph says: typedef='s2 l2 i', sizeof=16
        my $FLOCK_STRUCT = 's s l l i';

        sub linux_flock {
            if (wantarray) {
                my ($type, $whence, $start, $len, $pid) =
                    unpack($FLOCK_STRUCT, $_[0]);
                return ($type, $whence, $start, $len, $pid);
            } else {
                my ($type, $whence, $start, $len, $pid) = @_;
                return pack($FLOCK_STRUCT,
                        $type, $whence, $start, $len, $pid);
            }
        }

}

if (fork != 0) { # parent
    open(BLA, ">/tmp/bla") or die $!;
    chmod 02644, "/tmp/bla" or die $!;
    warn "lock ...";
    fcntl(*BLA, F_SETLK, linux_flock(F_WRLCK, SEEK_SET, 0, 0, 0)) or die $!;
    warn "lock done...";
} else {
    sleep 1;
    warn "try to open in other process";
    open(BLA, ">/tmp/bla") or die $!;
    warn "open was successful";
}