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

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: $!";