#        1         2         3       
#234567890123456789012345678901234567
$"='';grep{/^(([@_])(?!.*\2))+$/}<D>;

##</code><code>##

#234567890123456789012345678901234567
$"='';grep/^(([@_])(?!.*\2))+$/,<D>;

##</code><code>##

#        1         2         3         4         5         6         7         8
#23456789012345678901234567890123456789012345678901234567890123456789012345678901
$"='';$n{$_}++for@_;grep/^((??{"([@_])"})(??{"(?!([^$1 ]*$1){$n{$1},})"}))+$/,<D>

##</code><code>##

$"='';$n{$_}++for@_;grep/^(([@_])(??{"(?!([^$1 ]*$1){$n{$1},})"}))+$/,<D>