my $input = '';

until (($input =~ /^yes/i) or ($input =~ /^no/i)) {
  print "Please Enter Yes or No: \n";
  chomp($input = <>);
  printf "'%s' entered for system\n", $input;
[download]

That works. It'll echo every value entered, which you didn't ask for, but it will also prompt for "yes" or "no" after every 'incorrect' input. Why patterns instead of eq? Easier to customize, IMO, in case you want to allow for different case (with the /i, as above), or just allow for 'y' or 'n,' or whatever. HTH