my $email_template = get_email_template_as_string();

my $sql = 'SELECT first_name, last_name, email FROM address_book';
my $sth = $dbh->prepare_cached{$sql} or die 'Error';
while ( my ($first_name, $last_name, $email) = $sth->fetchrow_array() ) {
    my $text = $email_template;
    $text =~ s/__FNAME__/$first_name/;
    $text =~ s/__LNAME__/$last_name/;
    # contiue with substitutions
    send_email($text);
}