The short answer: you should be doing "libraries/somelib.pl" rather than requireing it. Reading over the docs for do (specifically, do EXPR, not do BLOCK... they are (sadly) not very closely related) and require would probably be a good idea, but the executive summary is: require will only execute it the first time it is seen in a perl process (because the expectation with require and use is that the file is a "package" or "module" and things are either exported or referred to by namespace... they don't have to be defined anew in each require-er).

Of course, you could also go the better route, and turn "libraries/somelib.pl" into a real module, and use it... but that will be more work (probably worth it, though).