Your test should be simply:

if ($Number!~/^[0-9]{1,8}$/)
{ 
## blah blah
}
[download]

..I'm surprised so many people are having a problem with this.

In English: For anything that doesn't explicitly match a string of numbers, and only numbers, of at least 1 character in length, but no more than 8 characters, do ##blah blah.

Cheers,
Bowie