Well this isn't going to make it any prettier (in fact, it is uglier), but if you have potentially undefined variables and you want to suppress the warnings, you should check to see if they're defined and then use them or not. Here's one example... obviously there are other ways to do it.

my $path = defined($dir) ? $dir : '' . defined($sub_dir) ? $sub_dir : 
+'' . defined($file) ? $file : '';
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