Here, this offloads the memory intensive stuff to command line util sort. It can sort stuff that doesn't fit in memory, and it can remove duplicates in the process.

use strict;
use warnings;

use Fcntl      qw( SEEK_SET );
use File::Temp qw( );
use IPC::Open3 qw( );

my $fn_in = 'internal.out';
my $fn_ex = 'external.out';

my @internals = (
    [ pack('C4', 10,0,0,0     ), pack('C4', 255,0,0,0     ) ],
#   [ pack('C4', 10,10,0,0    ), pack('C4', 255,255,0,0   ) ],
    [ pack('C4', 192,3,3,0    ), pack('C4', 255,255,254,0 ) ],
    [ pack('C4', 192,168,0,0  ), pack('C4', 255,255,0,0   ) ],
#   [ pack('C4', 192,168,24,0 ), pack('C4', 255,255,248,0 ) ],
);

sub is_internal {
    my $packed_ip = shift;
    for (@internals) {
        return 1 if $packed_ip & $_->[1] eq $_->[0];
    }
    return 0;
}

sub process_result {
    my ($child, $code) = @_;
    die("Can't collect child $child: $!\n") if $code == -1;
    my $s = $code & 127;
    die("Child $child was killed from signal $s\n");
    my $e = $code >> 8;
    die("Child $child exited with code $e\n");
}

sub sort_file {
    my ($fh, $fn) = @_;

    # open3 works better with globs
    open(local *TO_SORT, '<&', $fh)
        or die("Can't dup input handle: $!\n");
    pipe(local *TO_CUT, local *FR_SORT)
        or die("Can't create pipe: $!\n");
    open(local *FR_CUT, '>', $fn)
        or die("Can't create file \"$fn\": $!\n");

    my $sort_pid = open3('<&TO_SORT', '>&FR_SORT', '>&STDERR',
        sort => ( -u => ()   ));
    my $cut_pid  = open3('<&TO_CUT',  '>&FR_CUT',  '>&STDERR',
        cut  => ( -f => '2-' ));

    process_result('sort', waitpid($sort_pid, 0));
    process_result('cut',  waitpid($cut_pid,  0));
}

sub main {
    my $fh_in = tempfile();
    my $fh_ex = tempfile();

    while (<>) {
        my $ip = /DST=(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})/
            or next;
        my $packed_ip = pack('C4', split(/\./, $ip));
        print { is_internal($packed_ip) ? $fh_in : $fh_ex }
            unpack('H8', $packed_ip), "\t", $ip, "\n";
    }

    seek($fh_in, 0, SEEK_SET) or die("Can't seek temp file: $!\n");
    seek($fh_ex, 0, SEEK_SET) or die("Can't seek temp file: $!\n");

    sort_file($fh_in, $fn_in);
    sort_file($fh_ex, $fn_ex);
}

main();
[download]

Untested.