Here's one option:

use strict;
use warnings;
use File::Slurp qw/read_file/;

my ( %file1Hash, %file2Hash, %mergedHash );
my @files = sort { -M $a <=> -M $b } <"*.txt">;

do { chomp; $file1Hash{$_}++ }
  for read_file $files[0];
$mergedHash{$_}++ for keys %file1Hash;

do { chomp; $file2Hash{$_}++ }
  for read_file $files[1];
$mergedHash{$_}++ for keys %file2Hash;

print "$_\n" for grep $mergedHash{$_} == 1, keys %mergedHash;

[download]

What it does:

do { chomp; $file1Hash{$_}++ } for read_file $files[0];
       ^           ^            ^      ^
       |           |            |      |
       |           |            |      + - Read the file, returning a 
+list
       |           |            + - Do this for each line
       |           + - Make the line a hash key and increment the asso
+ciated value
       + - Remove the newline


$mergedHash{$_}++ for keys %file1Hash;
      ^            ^
      |            |
      |            + - Do this for each key
      + - Make the line a hash key and increment the associated value


print "$_\n" for grep $mergedHash{$_} == 1, keys %mergedHash;
        ^                              ^
        |                              |
        |                              + - If it only appears once (in
+ either file, but not both)
        + - Print the line

[download]

This uses a hash to tally identical lines, then shows only those keys (lines) whose value is 1, i.e., lines which only appear once in either file.

Hope this helps!

Update: Used three hashes: one for each file, and one merging the two hashes, in case the same line is repeated twice within a file--and those lines are only in that one file.

In reply to Re: compare most recent file with second most recent file and output difference by Kenosis
in thread compare most recent file with second most recent file and output difference by jjoseph8008

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