while(<DATA>) {
        print;
        s/\b([\do]+)\b/(my $x = $1) =~ s!o!0!gi; $x;/gie;
        print "--> $_";
}
__DATA__
lo123
asdf  lone fs
2oo
3O7
o123
o123b
321o
asf o987o 123 23o8
123     o4o4ooo