First, you might do well to read How do I post a question effectively? and Markup in the Monastery to improve the clarity and readability of your question. Well-written questions tend to get a much better response.

It sounds like you have a CSV file, for which you may want to use a CSV module. One straight-forward-to-use and well-vetted one is Text::CSV. While this may be a simple case, frequently it is the simple stuff that you have to keep returning to and reusing...

I do not see the matching you claim in the post. Is this simple equality, or is there a more subtle pattern? Assuming simple equality, this is a good opportunity to use hashes to organize your data. If you are unfamiliar with hashes, see Perl variable types. If you are having problems reading in the file in the first place, check out Files and I/O. One implementation that does what I think you want is: 

#!/usr/bin/perl
use strict;
use warnings;

my %data = ();
while (<DATA>) {
    my ($term1, $term2) = split;
    if (exists $data{$term2}) {
        print "$term2 found in both columns\n";
    }
    $data{$term1} = $term2;
}

__DATA__
NP_041954.1 NP_848263.1
NP_041955.1 NP_041956.1
I_match Not_here_though
I_dont_match I_match

[download]

Note I test before I store the value so that the entries must be in different rows (the spec as I understand it).

In reply to Re: how to compare column 1 to column 2 and vice versa from multiple rows. by kennethk
in thread how to compare column 1 to column 2 and vice versa from multiple rows. by BhariD

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