There are probably 50 or 60 ids.

Few enough!

The next line cannot match the one id. I should have posted a copy of that as well but basically 1 id will create 2 lines in the logfile and it is that second line I am trying to get at.

How 'bout the other question? Well, taking one assumption, one way to do what you're after may be along the lines of the following:

#!/usr/bin/perl

use strict;
use warnings;

die "Usage: $0 <id_file> [<files>]\n" unless @ARGV;

my %id_hit=map { substr($_, 0, 6) => 1 } do {
    my $idfile=shift;
    open my $fh, '<', $idfile or
      die "Can't open `$idfile': $!\n";
    <$idfile>;
};

while (<>) {
    if (/([A-Z]\d{5})/ and $id_hit{$1}) {
        defined(my $next=<>) or 
          die "No 'next' line in `$ARGV' at line $.\n";
        print $next;
    }
}

__END__
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