Load the first file into a hash instead of reading the a file over and over again.

And what's with using cat?!?!?! 

#!/usr/bin/perl

use strict;
use warnings;

my $File1 = '...';
my $File2 = '...';
my $File3 = '...';

my %keep;
{
   open(my $fh_keys, '<', $File1)
      or die("Can't open key file \"$File1\": $!\n);

   while (<$fh_keys>) {
      chomp;
      $keep{$_} = 1;
   }
}

{
   open(my $fh_in, '<', $File2)
      or die("Can't open input file \"$File2\": $!\n");

   open(my $fh_out, '>', $File3)
      or die("Can't create output file \"$File3\": $!\n");

   while (<$fh_in>) {
      my ($key) = /^[^|]*\|([^|]*)/;
      print $fh_out $_ if $keep{$key};
   }
}

[download]

Update: Added missing chomp as per reply.
Update: Added missing "$" in "my $fh_in" and "my $fh_out".

In reply to Re: Compare 2 files and create a new one if it matches by ikegami
in thread Compare 2 files and create a new one if it matches by shawshankred

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.