use Text::CSV_XS qw( csv );

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

my %fld = map { $_ => 1 } map { keys %$_ } @records;

csv (in => \@records, headers => [ sort keys %fld ]);

##</code><code>##

use Text::CSV_XS qw( csv );

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

my %seen;
my @fld = sort grep { !$seen{$_}++ } map { keys %$_ } @records;

csv (in => \@records, headers => \@fld, quote_empty => 1, on_in => sub {
    foreach my $f (@fld) {
        exists $_{$f} or $_{$f} = "--missing--";
        }
    });

##</code><code>##

AB1,MM,NN,XYZ
100,--missing--,200,400
200,300,--missing--,400
300,"",--missing--,