One way to do it is using Tie::File:

#!/usr/bin/perl

use strict;
use warnings;
use Tie::File;

my $file1 = '/path/to/file.txt';
my $file2 = '/path/to/file2.txt';

tie my @lines1,'Tie::File',$file1 or die $!;
tie my @lines2,'Tie::File',$file2 or die $!;
my @both = grep{my $i = $_; grep{$_ =~ $i}map{(split(/,/,$_))[1]}@line
+s2}map{(split(/,/,$_))[0]}@lines1;
untie @lines2;
untie @lines1;

print $_,"\n" for(@both);
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It's untested...