Thanks, but I've got lots and lots of these calculations, and what I'd like to be able to do is scope-globally adjust the behaviour, rather than have to wrap every block in either an eval or a conditional.

On reflection, a function mydiv:

sub mydiv {
  my ( $divided, $divisor ) = @_;
  $divisor ? $divided / $divisor : undef;
}

my $growth = mydiv($fig_2, $fig_1);
[download]

Might have been easier, but Abigail-II's answer is another way of getting to it as well.

Thanks
Alex