$ perl -le '
my $str = shift; 
print q{Original: }, $str; 
print q{Unique:   }, 
        grep { if ( $_ !~ m/(.).*?\g-1/ ) { $_; }; } ( $str, );
' foobar
Original: foobar
Unique:   
$ perl -le '
my $str = shift;
print q{Original: }, $str;
print q{Unique:   },
        grep { if ( $_ !~ m/(.).*?\g-1/ ) { $_; }; } ( $str, );
' bar
Original: bar
Unique:   bar