An alternative to me previous reply would be to use a hash instead of a regular expression 
my %pids;
{
    open(my $pids_fh, '<', $pids_qfn)
        or die("Can't open PIDS file \"$pids_qfn\": $!\n");

    while (<$pids_fh>) {
        my ($pid) = /\|(\d+)$/
            or next;
        ++$pids{$pid};
    }
}

open(my $fids_fh, '<', $fids_qfn)
    or die("Can't open FIDS file \"$fids_qfn\": $!\n");

while (<$fids_fh>) {
    my ($pid) = /^(\d+);/
        or next;
    print if $pids{$pid};
}

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In reply to Re^3: My code sucks, please help me understand why. (hash) by ikegami
in thread My code sucks, please help me understand why. by mirage4d

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