use Benchmark;

my @list;
for ( 0..9999 )
{
	push @list, sprintf "%d", 100 * rand ;
}


timethese(
	1000, 
	{
		'keys_map' => sub { my @uniq = keys %{{ map {$_ => 1} @list }}; },
		'grep_seen'	=> sub { my %seen; my @uniq = grep ! $seen{$_}++, @list; },
	}
);


##</code><code>##

Benchmark: timing 1000 iterations of grep_seen, keys_map...
 grep_seen: 13 wallclock secs (11.17 usr +  0.00 sys = 11.17 CPU) @ 89.52/s (n=1000)
  keys_map: 30 wallclock secs (29.28 usr +  0.00 sys = 29.28 CPU) @ 34.15/s (n=1000)