Here's one way to do this:

my $start = $ARGV[0];

while (1){
  print "Please Enter the Start-Level (0-7,?)[0]: ";
  chomp($start = <>);

  # Check for non-numeric characters
  if ($start =~ /\D/){
    print "\nPlease enter a number (0-7) or ? for help\n";
    next;
  }

  last if 0 <= $start and $start <= 7;

  # Check for a help request
  if ($start eq '?') {
    print " **** internally coded help information **** \n";
    next;
  }

  print "\nPlease enter a number between Zero and Seven, inclusive\n";
}

print $start;
[download]

First, we check that the input is numeric (as you started to do in your commented-out code) to prevent errors if the user enters non-numeric characters.
If the input is valid, we just exit the loop.
If not, check for a help request and, if not '?', print a generic error message and redo the loop.

There are a million different ways to do this, and this is only one of them...

Russ