use strict;
use Archive::Tar;
my $tar = Archive::Tar->new;

    #read "package.tar.gz" as a compressed file
$tar->read("package.tar.gz", 1); 

    #...

$file = "addfile.txt";

    #not using die here to prevent the "at pack.pl line x" part
open THEFILE, "<$file" or
    do{print "Could not open $file: $!\n"; exit 1}; 

    #get file contents
$data = {local $/ = undef; <THEFILE>};

close THEFILE or
    do{print "Could not close $file: $!\n"; exit 1};

    #create a file in the archive named after the original file, stored in directory pagepack.
    #copy the contents of the file to the archived file
$tar->add_files("pagepack/$file");
$tar->add_dat("pagepack/$file", $data); 

    #save the file as a compressed archive
    #This is where the checksum errors seem to appear
$tar->write("package.tar.gz", 1);