Perl module Text::Diff

use warnings;
use strict;

use Text::Diff;

my $text_diff = diff('a.txt', 'b.txt', {STYLE=>'Context'});

print $text_diff;
# *** a.txt       Wed Jul 30 16:07:18 2014
# --- b.txt       Wed Jul 30 16:09:45 2014
# ***************
# *** 1,9 ****
#   one two
#   three four five
# ! six seven eight
#   nine ten
#   eleven
# ! twelve thirteen fourteen
# ! fivteen sixteen
#   seventeen eighteen
#   nineteen twenty
# --- 1,9 ----
#   one two
#   three four five
# ! six SEVEN eight
#   nine ten
#   eleven
# ! twelve thirteen
# ! fourteen fivteen sixteen
#   seventeen eighteen
#   nineteen twenty

one two
three four five
six seven eight
nine ten
eleven
twelve thirteen fourteen
fivteen sixteen
seventeen eighteen
nineteen twenty

one two
three four five
six SEVEN eight
nine ten
eleven
twelve thirteen
fourteen fivteen sixteen
seventeen eighteen
nineteen twenty