package Foo;
use Readonly my $SPAM_FACTOR = 22; 
use Readonly my $SPAM_FUDGE  = 42;  # seasonally adjusted

# cause-cause-bundles-of-problems constructor
sub new { bless {spam=>4} , shift } 

# you can get or set bar with $that->bar
sub bar {
   my ($this, $newbar) = @_;
   if (@_) $this->{bar} = $newbar;
   
   return $this->{bar}; 
}

# you can't set baz, because it's derrived automatically from other attribute 
sub baz {
   my ($this) = @_;
   # just ignore @_ ...   perhaps warn "you cant change baz"
   return $this->{spam} * $SPAM_FACTOR + $SPAM_FUDGE;
}

# some time later
pacakge main;

my $foo = Foo->new();
$foo->bar( 'moose' ); # neat, bar is moose
$foo->baz( 'shoe' ); # not so much, worst of all, it looks right!

printf "I have a lovely foo, it has %s bar and %s baz " , $foo->bar, $foo->baz;
# hey! where's my shoe!?