$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/;
[download]

You are including the [ and ] characters but the OP only wants to change numerical digits.

    $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/;
[download]