Assumung users are entering letters, whitespace and other stuff, and you want only the first numbers (maximal 9), use

$number =~ s/.*?(\d{1,9}).*/sprintf "%09d", "$1"/e;
[download]

This does not work if users enters no digit. The best way to handle this is to perform not just a single check:

my $number = <STDIN>;
if( $number !~ /\d/ ) {
  print "no number";
}
else {
  # clean up and pad input
}
[download]

-- Frank