When you sort keys you are loading an array of keys to the memory, and sorting it, which can take up a lot again. I have the most perfect solution for you...

use strict;
use warnings;

use DB_File;

my $num_order_btree = new DB_File::BTREEINFO;
$num_order_btree->{compare} = sub { $_[0] <=> $_[1] };

tie my %h, 'DB_File', 'test_tie.tmp', O_RDWR|O_CREAT, 0640, $num_order
+_btree;
while (<INPUT>){
    $id = substr($_,9,11);
    unless (exists($h{$id})){
        $h{$id} = $_;
    } else { }
}

while (defined ($_ = each %h)){
    print OUTPUT $h{$_};
}

untie %h;
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-nuffin
zz zZ Z Z #!perl