A better try


use Data::Dimensions qw(&units extended);

my $width  = units({feet => 1},30);
my $length = units({feet => 1},50);
my $radius = units({inch => 1},10);
my $rate   = units({GBP => 1,
                    hectare => -1}, 1000);
my $cost   = units({GBP => 1});

set $cost = $rate *
    ($width*$length - 3.14159*$radius**2);

print "Cost is: $cost\n";