In this case, $prefix is not a global, because you've my'ed it. But I know what you're saying - you don't want to clutter your code with unneeded declarations.

The rule of thumb has to be that the best answer is the most fitting one. Does 'abcd' have any particular individual significance (so it might change in future), or is it just coincidence that it forms the start of all your strings (in which case it won't)?