Well, it all depends on how you setup your module.
If it is just a basic package that is not an object and does not \
use Exporter then you would call it like
RPG::DND::modifer(argument list here);
If you use the Exporter module when creating your module, then
you can (optionally) import the function name into the
current namespace and just call it is as
modifer(argument list);
If you make RPG::DND an object, you would create an object of
that type, something like my $rpg = new RPG::DND;, and then
you would call it like
$rpg->modifier(arguments);
Also, you might want to name it something like getModifer rather
than just modifier so it is slightly clearer on what the
subroutine does.