It's not a closure that you're using, because a closure "binds" to
lexical variables that exist *when the closure is created*. As
you said, $W doesn't exist then; it exists *later*.
So what you should probably do instead is just something like
this:
my $g = Games::Grid->new
(..., width => sub { $_[0]/4}, height => sub { $_[0] }, ...);
And then do:
$self->{terminal}->{WIDTH} = $config{width}->($W);
$self->{terminal}->{HEIGHT} = $config{height}->($H);
In other words, pass $W and $H as args rather than rely on them
being set when the sub is executed. If it were a closure, this
would work; but it's not, bceause you're relying on the lexicals
existing when you *execute* the subref, not when you *define*
it.
String eval might be another way of accomplishing your goal,
but I think you'd be better off just passing args.