Read the files line-by-line, split each pair of lines along whitespace, compare the relevant field, and output the lines if they differ: 

#!/usr/bin/perl

use feature qw/say/;
use warnings;
use strict;

open my $file1, "<", "test1.txt" or die "Could not open first file: $!
+\n";
open my $file2, "<", "test2.txt" or die "Could not open second file: $
+!\n";

while(my $line1 = <$file1>) {
  my $line2 = <$file2>;

  chomp ($line1, $line2);

  my ($key1, $num1, $str1) = split /\s+/, $line1;
  my ($key2, $num2, $str2) = split /\s+/, $line2;

  if($str1 ne $str2) {
    say $line1;
    say $line2;
  }
}

close $file1 or die "Could not close first file: $!\n";
close $file2 or die "Could not close second file: $!\n";

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There's bound to be shorter, more idiomatic ways of achieving the same thing, but since your userpage indicates you're still new to Perl, you may find this the most instructive/useful.

On an unrelated side note, could you edit your post to use <code> tags for your sample data? See the following nodes for more information on formatting etc.:

In reply to Re: Sorting By Column by AppleFritter
in thread Sorting By Column by Roguehorse

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
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