use strict;
use warnings;

use Data::Dumper;

my $str1 = 'Perlmonks is the best perl community';
my $str2 = 'Perlmonks is one of the best community of perl users';

if ($str1 eq $str2) {
	print $str1;
	exit;
}

my @wl1 = split /\s+/, $str1; 
my @wl2 = split /\s+/, $str2; 

my $wp1 = build_word_hash(\@wl1);
my $wp2 = build_word_hash(\@wl2);


my $diff_str1 = '';
my $diff_str2 = '';

while (@wl1 || @wl2) {
	my $word1 = shift @wl1;
	my $word2 = shift @wl2;


	if ($word1 && $word2 && $word1 eq $word2) {
		$diff_str1 .= $word1 . ' '; # pairing the word from the origional string with it's output 
		$diff_str2 .= $word2 . ' '; # lets us do things like case insensitive, but preserving match later
		shift @{$wp1->{$word1}}; # eat this word
		shift @{$wp2->{$word2}}; # eat this word
		next;
	}
	#process word1 first, for fun
	if ($word1) {
		if ($wp2->{$word1} 
			&& @{$wp2->{$word1}} 
			&& ! grep {$_ == $wp2->{$word1}->[0]} @{$wp1->{$word1}} ) {
			# word moved.
			# the grep checks that the next occurance of the word in string 2 ($wp2->{$word}->[0]
			# does not also have an occurance of the word in string 1.
			# if it does not, it means that this is a move of the word.
			$diff_str1 .= "[$word1] ";
			
			shift @{$wp2->{$word1}}; # eat this word
		}
		else {
			# Easy case, word in string 1 but not string 2
			$diff_str1 .= "<$word1> ";
		}
	}

	 if ($word2) {
		 if ($wp1->{$word2} && @{$wp1->{$word2}} && ! grep {$_ == $wp1->{$word2}->[0]} @{$wp2->{$word2}}  ) {
			$diff_str2 .= "[$word2] ";
			
			shift @{$wp1->{$word2}}; # eat this word
		}
		else {
			$diff_str2 .= "<$word2> ";
		}
	}
}

print "$diff_str1\n$diff_str2\n";

sub build_word_hash {
	my $wl = shift;
	my $res = {};
	my $i = 0;
	foreach my $word( @$wl ) {
		push @{$res->{$word}} , $i++;
	}
	return $res;
}

##</code><code>##

Perlmonks is [the] [best] [perl] [community]
Perlmonks is <one> <of> [the] [best] [community] <of> [perl] <users>