my $AoH;

for my $n_rec (1, 10, 100, 1000) {
    say "";
    say "=== num of records is: ", $n_rec;
    $AoH = create_data (1, $n_rec);
    is_deeply ([ sort $_->() ], [ sort &slice_join ]) for \&list_join,
+ \&grep_map, \&map_map;

    cmpthese ( -1, {
        map_map    => \&map_map,
        grep_map   => \&grep_map,
        list_join  => \&list_join,
        slice_join => \&slice_join,
        });

    }

done_testing;

sub list_join {
    my %H;
    %H = (%H, %$_) for @$AoH;
    return keys %H;
    } # list_join

sub slice_join {
    my %H;
    @H{keys %$_} = () for @$AoH;
    return keys %H;
    } # slice_join

sub grep_map {
    my %seen;
    return grep { !$seen{$_}++ } map { keys %$_ } @$AoH;
    } # grep_map

sub map_map {
    my %H = map { $_ => 1 } map { keys %$_ } @$AoH;
    return keys %H;
    } # map_map

sub create_data {
    my ($density, $records) = @_;

    my @AoH;
    push @AoH, {map { rand 100 <= $density ? ("$_" => $_) : () } "A" .
+. "ZZ"}
        for 1 .. $records;
    return \@AoH;
    }
[download]

=== num of records is: 1
ok 1
ok 2
ok 3
               Rate    map_map  list_join   grep_map slice_join
map_map    508970/s         --       -18%       -19%       -45%
list_join  619376/s        22%         --        -1%       -33%
grep_map   625570/s        23%         1%         --       -33%
slice_join 928647/s        82%        50%        48%         --

=== num of records is: 10
ok 4
ok 5
ok 6
               Rate  list_join    map_map   grep_map slice_join
list_join   12444/s         --       -80%       -83%       -88%
map_map     61440/s       394%         --       -17%       -42%
grep_map    73770/s       493%        20%         --       -30%
slice_join 105217/s       746%        71%        43%         --

=== num of records is: 100
ok 7
ok 8
ok 9
              Rate  list_join    map_map   grep_map slice_join
list_join    174/s         --       -97%       -98%       -98%
map_map     5966/s      3332%         --       -25%       -43%
grep_map    7952/s      4474%        33%         --       -24%
slice_join 10479/s      5928%        76%        32%         --

=== num of records is: 1000
ok 10
ok 11
ok 12
             Rate  list_join    map_map   grep_map slice_join
list_join  6.67/s         --       -99%       -99%       -99%
map_map     545/s      8070%         --       -47%       -56%
grep_map   1027/s     15299%        88%         --       -17%
slice_join 1244/s     18553%       128%        21%         --
[download]