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

while (<OLD>) {
    # change $_, then ...
    print NEW $_               or die "can't write $new: $!";
}

close( OLD )                   or die "can't close $old: $!";
close( NEW )                   or die "can't close $new: $!";

rename( $old, "$old.orig" )    or die "can't rename $old to $old.orig: $!";
rename( $new, $old )           or die "can't rename $new to $old: $!";

##</code><code>##

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: $!";