# all in quotes
$ack = "$config{'startblock'}<rest of ACK msg>$config{'endblock'}";

# encap chars outside of quotes
$ack = $config{'startblock'} . "<rest of ACK>" . $config{'endblock'};

# regex
$ack = "<ack msg here>";
$ack =~ s/(.*)/$config{'startblock'}\1$config{'endblock'}/;