> after %H = (%H,%$_) for @AoH you'll have the superset in keys %H

actually using slices and empty key lists is way faster

@H{keys %$_} = () for @$AoH;

DEMO: 

use v5.12;
use warnings;
use Test::More;
use Benchmark qw/cmpthese/;

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 &list_join],
              [sort &slice_join],
             );

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

}

done_testing;

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

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

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

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

[download]
OUTPUT: 
__DATA__

=== num of records is: 1
ok 1
               Rate  list_join slice_join
list_join  238532/s         --       -65%
slice_join 682713/s       186%         --
__DATA__

=== num of records is: 10
ok 2
               Rate  list_join slice_join
list_join    7819/s         --       -93%
slice_join 112993/s      1345%         --
__DATA__

=== num of records is: 100
ok 3
             Rate  list_join slice_join
list_join  82.9/s         --       -99%
slice_join 8533/s     10195%         --
__DATA__

=== num of records is: 1000
ok 4
             Rate  list_join slice_join
list_join  3.66/s         --      -100%
slice_join 1067/s     29072%         --
1..4

[download]

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

Update

fixed bug in sorted tests

In reply to Re^2: Writing hashes as records to a CSV file (joining keys with slices) by LanX
in thread Writing hashes as records to a CSV file by Serene Hacker

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