$ perl -E '
    my $str = "sssDDDsssDDDssUss";
    my ($fore, $aft) = split /U/, $str, 2;
    $fore =~ s/s/i/g;
    $aft =~ s/s/o/g;
    say "$str\n", join "U", $fore, $aft;
'
sssDDDsssDDDssUss
iiiDDDiiiDDDiiUoo

##</code><code>##

$ perl -E '
    my $str = "sssDDDsssDDDssUss";
    say $str;
    $str =~ y/DU/M/;
    say $str;
'
sssDDDsssDDDssUss
sssMMMsssMMMssMss