>> would the use lib $g_moduleDir;give a runtime error?

Yes. use lib $g_moduleDir would need to be processed at RUNTIME after the $g_moduleDir variable has been set. Unfortunately "use" is processed at compile time and as such $g_moduleDir has not been set which will always create the empty string error that you have seen. You can hard code the path and it will work  use lib 'path/to/my/personal/lib/dir'

*Update:

Amusingly enough you can do something like this

#!/usr/bin/perl
BEGIN{
  use Config::YAML;
  our $c= Config::YAML->new("/pathtoconfigfile");
  our $inc_path = $c->get_includePath;
}
use lib $inc_path;
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