You can collect the column names by iterating over the records and accumulating their keys.

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use List::Util qw{ uniq };
use Text::CSV_XS;

my @records = ({AB1 => 100, NN => 200, XYZ => 400},
               {AB1 => 100, XYZ => 400, MM => 300});

my @columns = sort +uniq(map keys %$_, @records);
my $csv = 'Text::CSV_XS'->new({auto_diag => 1});
for my $record ({map { $_ => $_ } @columns}, @records) {
    $csv->say(\*STDOUT, [@$record{@columns}]);
}
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