Do you want to check:

• Which keys are different in both files; or
• Which values are different (for corresponding keys); or
• A combination of both.

To expand a little on what CountZero said, It's not entirely clear what you're trying to do, but since you already started working on this, you could make it easier to help you if you did a few things:

• Post your complete code so far
• Post a short set of example input
• Show the output you want
• Show the output you're getting

If you wrap it all in <code> tags it will be easy to read and you'll probably get one or more solutions pretty quickly.

Hello mimiandi,

While I agree that your OP needs clarification and supplemental code, here is a simple method using grep to gather keys that differ.

#!/usr/bin/perl

use strict;
use warnings;

my %file1 = ( samp1 => 'stuff',
              samp2 => 'data',
              samp3 => 'info', 
              samp4 => 'foo',
              samp5 => 'bar',
              blah  => 'blah',
              brood => 'other' );  

my %file2 = ( samp1 => 'stuff',
              samp2 => 'diff',
              samp3 => 'blah',
              samp4 => 'foo',
              junk  => 'stuff',
              trash => 'other',
              samp5 => 'bar' );

my (@diff1, @diff2);

push @diff1, grep !$file2{$_}, keys %file1;
push @diff2, grep !$file1{$_}, keys %file2;
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Depending on the complexity of your data, you may want to have a look at Data::Compare. I believe Test::More also offers some tools for comparing data structures.

Post some code or at least some sample input data and I'm sure you'll rope some better answers.

Suggested Reading: How (Not) To Ask A Question

It appears that the "key" is the entire line of file1. If so, then this is a pretty easy task.

Here is an example:

use Modern::Perl;

my %keys;

open my $file1, '<', 'filename1.txt' or die "Can't open file 1: $!";
open my $file2, '<', 'filename2.txt' or die "Can't open file 2: $!";

#read in the first file.
while (<$file1>)
{
    chomp;             #remove newline.
    $keys{$_}++;    #Add the key to your hash
}

#find keys in the second file.
while (<$file2>)
{
    foreach my $key (keys %keys)
    {
        say "$key matches" if /\b\Q$key\E\b/;
    }     
}
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Regex Notes: \b matches a word boundary. This is because you presumably wouldn't want "John Smith" to match "John Smithfield". \Q...\E is the quote literal modifier. If your key contains characters with a special meaning in regexes (such as the dot in "Mr. Smith") you want to match only the literal characters.

Yes, you do end up needing a nested loop...that is intrinsic to the nature of your task.