Taking poj's nice code to use filehandle: 
use strict;
use warnings;
use autodie;

my %data=();
my $file_one = 'test.txt';
my $file_two = 'test2.txt';

open ( my $fh1, '<', $file_one);
open ( my $fh2, '<', $file_two);

my @data1 = <$fh1>;
my @data2 = <$fh2>;

for (@data1,@data2){
  my ($sz,$sum,@f) = split /\s+/;
  push @{$data{$sz}},$_ for @f;
}

for my $sz (sort {$a <=> $b} keys %data){
  print join ' ',@{$data{$sz}},"\n";
}

[download]
UPDATE: if the data files actually contain new lines, chomp($_) in the first for loop

In reply to Re^2: Appending arrays into the rows of a 2 dimension array by PerlSufi
in thread Appending arrays into the rows of a 2 dimension array by SoftwareGoddess

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