my @a = (0..1000, 0..1000);
Benchmark::timethese($ARGV[0]||-1,{
  "shift" => sub {
     my (%seen, $string);
     my @array = @a;
     $seen{$string}++ while defined ($string = shift @array);
     for (keys %seen) {
        push @array, $_;
        delete $seen{$_};
     } return @array;
  },
  "grep"  => sub {
     my %seen;
     my @unique = grep { ! $seen{$_}++ } @a;
  },
});
__END__
      grep:  1 wallclock secs ( 1.04 usr +  0.00 sys =  1.04 CPU) @ 614.42/s (n=639)

     shift:  1 wallclock secs ( 1.03 usr +  0.00 sys =  1.03 CPU) @ 296.12/s (n=305)