file lock and update

[CColin]

Hi, I have a program which modifies a file in place using a temporary file

open(OLD, "< $old")         or die "can't open $old: $!";
open(NEW, "> $new")         or die "can't open $new: $!";
my @array = <OLD>;
my $required_element_details = shift(@array);

foreach my $remaining_details (@array) {
print NEW "$remaining_details";
}

close(OLD)   
close(NEW)                 

rename($new, $old)

Problem is this file can be accessed at the same time by the same program so the reading and writing process needs to be locked. Have tried:

open(OLD, "< $old")         or die "can't open $old: $!";
open(NEW, "> $new")         or die "can't open $new: $!";
flock (OLD, 2);
flock (NEW, 2);

[continue as before including renaming the file before the filehandles
+ are closed]

But this doesn't work when both programs are trying at the same time, instead both programs read the same initial element (and write the same results). Any suggestions? Colin