$begin .= substr $num, 0, 1, q{} until       7 == $begin =~ tr/[0-9]//;
...
    $end = substr($num, -1, 1, q{}) . $end until 5 == $end   =~ tr/[0-9//;
...
    $num =~ tr/[0-9]/X/;

##</code><code>##

    $begin .= substr $num, 0, 1, q{} until       7 == $begin =~ tr/0-9//;
...
    $end = substr($num, -1, 1, q{}) . $end until 5 == $end   =~ tr/0-9//;
...
    $num =~ tr/0-9/X/;