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#!/usr/local/bin/perl -ln0777

...

  print " " x (40 - ($c = 1 + length $1) / 2), $1
}

                                      a 
                                    a a a 
...
                      words only on spaces or new lines

> perl -ln0777we '$c=0;s/\s+/ /sg;print" "x(40-($c=length$1)/2),$1 whi
+le s/(.{$c,}?. |.+$)//' a.txt w.txt
                                       a 
                                      a a 
...
                                     lines

> perl -ln0777e's/\s+/ /sg;print$"x(40-($c=length$&)/2),$&while s/.{0$
+c,}?. |.+$//' a.txt w.txt
                                       a 
                                      a a 
...
                                a a a a a a a a 
                                   a a a a a