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);
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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);
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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);
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Also, you might want to name it something like getModifer rather than just modifier so it is slightly clearer on what the subroutine does.