my $zones = reduce {
    my( $first, $last ) = split '-', $b;
    $last ||= $first;
    
    #allocate $a here
    $a .= chr(0) x ( $last );
    
    substr( $a, $first, $last - $first + 1 ) = 'x' x ( $last - $first + 1 );
    $a
} '', split ',', 
'10-374,376-379,382-385,388-499,530-534,541-543,618,619,700-704,707-709';