#!/usr/bin/perl -w
use strict;
use Data::Dumper;
$|++;

my @AoA = ( undef,
            [1, 2],
            undef,
            [3, 4]);
          
my @flat = map{defined (@$_) ? @$_ : ()}@AoA;

print join (",",@flat),"\n"; # prints: 1,2,3,4

my $AoAref = \@AoA;

@flat = map{ defined (@$_) ? @$_ : () }@$AoAref;
print join (",",@flat),"\n"; # prints: 1,2,3,4

=so far prints:...
1,2,3,4
1,2,3,4
=cut

@AoA = ( undef,
         [1, undef],
         undef,
         [3, 4]);
$AoAref = \@AoA;         

# "undef" is a legal Perl value.
# to handle this idea of an undef value within
# an defined array (undef is a legal Perl value)! Yes, use grep.
#
# my map filters out "undefined references to arrays"
# within an AoA, I would use grep to filter 
# that undesired "undef" value out if what is meant
# a value within in an array.
# as shown before: grep{} map{}.