I don't understand what's wrong.

It's clear that the example data you gave us doesn't give any matches, but if I modify it a bit I do get a match.

Code: 

use strict;
use warnings;
use autodie;


open my $pids, '<', 'pids';
my %ids;
while (my $line = <$pids>) {
    chomp $line;
    my (undef, $id) = split /\|/, $line;
    $ids{$id} = 1;
}
close $pids;

open my $fids, '<', 'fids';
while (my $line = <$fids>) {
    my ($id, $rest) = split /;/, $line, 2;
    if ($ids{$id}) {
        print "Found id '$id' in line $line";
    }
}
close $fids;

[download]

FIDS: 

810000001;08/17/1957
810000002;12/02/1975
810000003;12/22/1982
810000004;11/01/1967
810086196;02/07/1981
810000006;12/27/1967
810000007;05/09/1981
810000008;05/14/1976
810000009;10/24/1981
810000010;11/17/1943

[download]

pids (unchanged): 

dn: cn=*****,ou=users,|810221480
dn: cn=*****,ou=users,|810039655
dn: cn=*****,ou=users,|810086196
dn: cn=*****,ou=users,|810008482
dn: cn=*****,ou=users,|810224329

[download]
Perl 6 - links to (nearly) everything that is Perl 6.

In reply to Re^5: My code sucks, please help me understand why. by moritz
in thread My code sucks, please help me understand why. by mirage4d

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.