johngg@shiraz:~/perl/Monks > perl -Mstrict -Mwarnings -MData::Dumper -E '
my $data = [
   [ 123 ],
   [ 789 ],
   [ 456 ],
   [ 123 ],
   [ 543 ],
   ];

my @uniq = do {
   my %seen;
   grep { not $seen{ $_->[ 0 ] } ++ }
   @{ $data };
   };

print Data::Dumper->Dumpxs( [ \ @uniq ], [ qw{ *uniq } ] );'
@uniq = (
          [
            123
          ],
          [
            789
          ],
          [
            456
          ],
          [
            543
          ]
        );