use Archive::Tar;

$tar = Archive::Tar->new();
$tar->read("origin.tar.gz",1); # you don't seem to want this line though
$tar->add_files("file/foo.c", "file/bar.c");
$tar->add_data("file/baz.c","This is the file contents");
# $tar->write("files.tar");
print STDOUT $tar->write(); # make sure http headers are already printed before running this line