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: $!";