I can shorten your 36 non-duplicate-letter attempt by 5 chars with:

#        1         2         3
#234567890123456789012345678901
grep/^(([@_])(?!.*\2))+$/x,<D>;
[download]

I think this trick might be applicable to your others as well...

Update: I'm still trying to grok your longer ones (re eval in golf... coool) and I'm uncovering some oddities:

qw(a d d) 
   ad
   add
   dad

qw(a d)
   ad
   add  <= wrong... but 'dad' got correctly skipped... I think
                    add gets through because the 'd's are right
                    next to each other
[download]

Doubled letters (dd, tt, etc) are sneaking through... It looks like $n{'d'} needs to be temporarilly decremented when 'd' matches in the character class...

-Blake