sub LTR {
    # 116 characters
    my@a;for(@_){if(/\d/){push@a,$_}else{$/=pop@a;
    $,=join' ',pop@a,$_,$/;$,=~s/(.*)/($1)/ if((/[+-]/);push@a,$,;$;=$
+_}}@a
}
print LTR qw(1 2 - 3 4 + 5 * /);
[download]

My solution don't really minimize brackets :( (but seems to be correct most of the time; more brackets are better than less :))

Another problem is that I don't use %operator because it wasn't really helpful (without last change). So It can't solve Update #2 from Masem (I only see it now).

This problem is very tricky... When should we use brackets or not ???
Congratulation to chipmunk and MeowChow for their contributions.

BobiOne KenoBi ;)