my $SIZE = 1000;
sub create_matrix {
    my ($filename) = @_;
    open my $OUT, '>', $filename or die $!;

    for my $i (1 .. $SIZE) {
        for my $j ( 1 .. $SIZE ) {
            print {$OUT} $i <= $j ? $i * $j : 'NA';
            print {$OUT} ' ' unless $SIZE == $j;
        }
        print {$OUT} "\n";
    }
}