perl -nle"my %seen; print grep !$seen{$_}++, /./g"

##</code><code>##

perl -nle"my %seen; print sort grep !$seen{$_}++, /./g"

##</code><code>##

>perl -MO=Deparse -nle"foo()"
BEGIN { $/ = "\n"; $\ = "\n"; }
LINE: while (defined($_ = <ARGV>)) {
    chomp $_;
    foo();
}
-e syntax OK