( [ @array ] ) x @array ;
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Make N copies of [ @array ]

@array{ @array } = ( [ @array ] ) x @array ;
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Do the hash assign of the N copies. This is all obvious so far, now for the trickier bit, the output

%array = (
           'c' => [
                    'a',
                    'b',
                    'c',
                    'd'
                  ],
           'a' => $array{'c'},
           'b' => $array{'c'},
           'd' => $array{'c'}
         );
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Unusual, you say, that the key 'c' should get 'the original' and the rest get copies, but if we look at the ordering of the keys this makes sense

shell> perl -le 'print keys %{{map {$_=>1} qw/a b c d/}}'
cabd
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So as you may have guessed by now Data::Dumper's output shows that 'c' just happens to look like its got 'the original' due to the ordering of the hash. In reality they all point to the same anonymous array.
HTH

_________
broquaint