My variation using csv:

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 ]);

[download]

A suggestion might be to add quote_empty => 1 to be able to distinguish between missing keys and defined keys. But as that does not shows the difference between missing keys and keys that point to an undefined value, I would suggest something like this (unless you don't care about those differences):

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--";
        }
    });

[download]

->

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

[download]

To answer LanX' question: as CSV is streaming, you don't know what is to come, so there is no builtin functionality to find unique values in CSV, but there are enough builtin features to do it yourself.


Enjoy, Have FUN! H.Merijn

In reply to Re: Writing hashes as records to a CSV file by Tux
in thread Writing hashes as records to a CSV file by Serene Hacker

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.