#!/usr/bin/perl
use strict;  
use warnings;  

my $master = 'master.txt';
my $newmaster = 'master.tmp';
my $update = 'update.txt';

open my $updatefh, '<', $update or die "$update: $!";

my %updates;
my @updates;
foreach my $line (<$updatefh>) {
    my $key = substr($line,3,12);
    $updates{$key} = $line;
    push(@updates, $key);
}
close($updatefh);

open my $masterfh, '<', $master or die "$master: $!";
open my $newmasterfh, '>', $newmaster or die "$newmaster: $!";

foreach my $line (<$masterfh>) {
    my $key = substr($line,3,12);
    if(exists($updates{$key})) {
        print $newmasterfh $updates{$key};
        delete($updates{$key});
    } else {
        print $newmasterfh $line;
    }
}
close($masterfh);

foreach my $key (@updates) {
    if(exists($updates{$key})) {
        print $newmasterfh $updates{$key};
    }
}
close($newmasterfh);