What you do with $c->set_area( -42 )?

Maybe I'm missing something. Say that I implemented set_area as

sub set_area { 
    my $self = shift; 
    my $area = shift; 
    die "Area must be non-negative." unless $area >= 0;
    # ... 
}
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How do you go back and change the interface later to avoid set_foo methods?

You don't. But, that's okay because you never have a need to.

-sauoq
"My two cents aren't worth a dime.";