use Benchmark; my @list; for ( 0..9999 ) { push @list, sprintf "%d", 100 * rand ; } timethese( 1000, { 'keys_map_1' => sub { my @uniq = keys %{{ map {$_ => 1} @list }}; }, 'keys_map_undef' => sub { my @uniq = keys %{{ map {$_ => undef} @list }}; }, 'grep_seen' => sub { my %seen; my @uniq = grep ! $seen{$_}++, @list; }, } ); __END__ Benchmark: timing 1000 iterations of grep_seen, keys_map_1, keys_map_undef... grep_seen: 15 wallclock secs (14.86 usr + 0.01 sys = 14.87 CPU) @ 67.23/s (n=1000) keys_map_1: 50 wallclock secs (46.78 usr + 0.83 sys = 47.61 CPU) @ 21.00/s (n=1000) keys_map_undef: 43 wallclock secs (42.16 usr + 0.94 sys = 43.09 CPU) @ 23.21/s (n=1000)