$str1 = "12345 ABC 987 MNO";
$str2 = "      CDE";

##</code><code>##

$str = "12345 CDE 987 MNO";

##</code><code>##

my @arr1 = split //, $str1;
my @arr2 = split //, $str2;
foreach (my $i=0;$i<@arr1;$i++) {
        if ($arr2[$i]) { # we do have something in the second string
                if (($arr1[$i] ne " ") and ($arr2[$i] eq " ")) {
                        $arr2[$i] = $arr1[$i];
                }
        } else { #$str2 is empty now, so copy all
                $arr2[$i] = $arr1[$i];
        }
}
print join "",@arr2,"\n";