#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my %HoA = ( N1 => [undef],
	    N2 => [undef],
	    N3 => [undef],
	    N4 => [undef] );
print Dumper \%HoA;

# add to an existing row
push @{ $HoA{"N1"} }, "A", "B";
print Dumper \%HoA;

__END__

$ perl test.pl
$VAR1 = {
          'N3' => [
                    undef
                  ],
          'N2' => [
                    undef
                  ],
          'N1' => [
                    undef
                  ],
          'N4' => [
                    undef
                  ]
        };
$VAR1 = {
          'N3' => [
                    undef
                  ],
          'N2' => [
                    undef
                  ],
          'N1' => [
                    undef,
                    'A',
                    'B'
                  ],
          'N4' => [
                    undef
                  ]
        };