perl -ln0e's/\s+/ /g;print$"x(40-($c=2+length$&)/2),$&while/.{0$c,}? |.+/g' a.txt w.txt
                                      a 
                                    a a a 
                                  a a a a a 
                                a a a a a a a 
                              a a a a a a a a a 
                             a a a a a a a a a a 
                          The same principle applies 
                    to any text file, basically splitting 
                      words only on spaces or new lines 


perl -ln0e's/\s+/ /g;print$"x(40-($c=2+length$&)/2),$&while/.{0$c,}? |.+/g' w.txt a.txt
                                     The 
                                same principle 
                             applies to any text 
                          file, basically splitting 
                      words only on spaces or new lines 
                    a a a a a a a a a a a a a a a a a a a 
                       a a a a a a a a a a a a a a a a