Here's one very perlish way to do it, and remember to test for your `sanity' conditions - in your above code you are assuming that each input line is an even multiple of 320 characters long - the very least you should do is die() if that is not the case.

open IN, $file
    or die "Cannot open $file for reading; $!";
my $c = 0;
my @lines = map {
    $c++;
    chomp;
    die "Input line $c in $file irregular:\n$_\n"
        if (length % 320);
    m/(.{320})/g
} <IN>;
close IN;

if (@lines) {
    open OUT, ">$file.new"
        or die "Can't open $file.new for output; $!";
    print $_,"\n" foreach (@lines);
    close OUT;
}
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