my $Usage = <<ENDUSE;
Usage: $0 N
  where N is a number between 1 and 10:
  1 : Alzheimer disease
  2 : Bardet Biedl
  3 : Breast cancer
  ...
ENDUSE

die $Usage unless ( @ARGV == 1 and $ARGV[0] =~ /^\d+$/ 
                    and $ARGV[0] > 0 and #ARGV[0] < 11 );
my $choice = shift;

...