open( my $fh, '/usr/share/dict/propernames' ) or die $!;
while (<$fh>) {
    chomp;
    push( @{$words{length($_)}}, $_ );
}

while (<DATA>) {
    s/(\S+)/replace($1)/ge;
    print;
}

sub replace {
    my $list = $words{length $_[0]} or return $_[0];
    return $list->[rand @$list];
}

__DATA__
.,        ,; .r,
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 nq      aq,
 Brg    iaB  WiW  iqpbduk:   ifcciWvj  Wypdip  @rnoqqS
  lc    st   unx  mm    su. Wyl.  eee  daa    jb;   @kS
  kjt  smp   WkW    8hytct: ih     xd  WiZ      Zlantc@
   tg .vk    WrW  cyW:: hy, vx     bo  WnW    gtx,i 0rW
   SnjsaS    WbW gw     oo  kkZ   rto  WeW   fvB    0qZ
    xbcd     ocg  tfrotxynk  veqWhurb  kdy    wavkuseax0
              ,    ,:;i   :        yr   ,       ,;i   .:
                            Zjc   ugr
                              btfau.