my %default_fields =(
  desired_temp =>4,
  desired_volume=>qw[/1 2 3 4 5/],
  desired_mark=>100
);
my @required_fields=qw/location/;
my @fields =(keys %default_fields,@required_fields);
#I don't quite get what the part keys %default_fields) .. I # know it put and expand everything into the @fields array

sub new {
 my ($class,%args)=@_;
 foreach (@required_fiels) {
     #what does exists($args{$_}}means\?
     exists($args{$_}) or croak('required fieds $_ omitted");
  }
  %args = (%default_fields,%args); #either set to defaults or new values
  @this{@fields} = @args{@fields};
  #this create a this array of array and set the contents to args
  return bless(\%this,$class); 
 
}