Perhaps this will help get you started:

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


my $data1 = 'master.txt';
my $data2 = 'child.txt';


open my $F1, '<', $data1 or die qq["$data1" can\'t be opened: $!];
open my $F2, '<', $data2 or die qq["$data2" can\'t be opened: $!];


my %accounts;
while ( <$F1> ) {
    my ( $number, $name ) = unpack 'A4 A*', $_;
    $accounts{ $number }{ master } = $name;
    }

while ( <$F2> ) {
    my ( $number, $child ) = unpack 'A4 A*', $_;
    $accounts{ $number }{ child } .= "$child\n";
    }

my $page = 1;
for my $key ( sort keys %accounts ) {
    print ' ' x 20, $accounts{ $key }{ master }, "\n";
    print $accounts{ $key }{ child };
    print ' ' x 20, "Page ", $page++, "\n\n";
    }
[download]