use Some::Module;
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can be inlined as

BEGIN {
   ... Contents of Some/Module.pm ...

   $INC{'Some/Module.pm'} = 1;
}

use Some::Module;
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(There are differences, such as lexical pragmas before the BEGIN block will also affect the BEGIN block, but it's a good starting point.)

So what you want:

use strict;
use warnings;
use diagnostics;
use 5.014;

BEGIN {
   package Lib::Book;

   use Moo;

   has isbn => (
      is => 'rw',
   );

   $INC{"Lib/Book.pm"} = 1;
}

use Lib::Book;

my $newbook = Lib::Book->new(
    isbn => 'ISBN',
);

say $newbook->isbn;
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