Indeed. And the reason [^\1] doesn't work is... well because within a character class \1 means 1, not the contents of the capture buffer 1. Which is a long way of saying it doesn't work because it doesn't work (however plausible it may look).

I eventually ended up with the following:

my $unique = qr/^(.)(?!\1)
                 (.)(?!\1|\2)
                 (.)(?!\1|\2|\3)
                  .$/x;
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   larryk                                          
perl -le "s,,reverse killer,e,y,rifle,lycra,,print"