If a standard does not work for you, why don't you just disable it?

Also, you could cobble together the class ABC by importing subroutines from ABC::stage1, ABC::stage2 and ABC::stage3 into it:

package ABC;
use ABC::stage1 ':all';
use ABC::stage2 ':all';
use ABC::stage3 ':all';

1;
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... or you could use a version control system that has good merging, like for example Git.

I recommend the last option.