use Fcntl qw( :flock :DEFAULT );
sysopen( OUT, $out, O_WRONLY|O_CREAT ) or die "can't sysopen $out: $!";
flock( OUT, LOCK_EX )                  or die "can't flock $out: $!";
truncate( OUT, 0)                      or die "can't truncate $out: $!";
# now write to file...
close( OUT )                           or die "can't close $out: $!";