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 ]);