#!/usr/bin/perl

# http://perlmonks.org/?node_id=1141535

use strict;
use warnings;

my $numwords = 40;

$_ = "To be or not to be.\n"; # or read from file...

my @previous; # holds the $numwords previous words
s/(\w+)/
  my $match = grep lc $1 eq lc, @previous;
  push @previous, $1;
  @previous > $numwords and shift @previous;
  $match ? "*$1*" : $1 /ge;

print;