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--";
        }
    });