salva made the good that you're not looking to sort, so much as group. You don't need sort to do that. 

my %conversations;

while (<DATA>) {
   my ($src, $dst) = (split)[0, 2];
   $src = pack('C4n', split(/[.:]/, $src));
   $dst = pack('C4n', split(/[.:]/, $dst));
   my $key = $src lt $dst ? "$src$dst" : "$dst$src";
   push(@{$conversations{$key}}, $_);
}

foreach (values %conversations) {
   print(@$_);
}

[download]
outputs 
10.10.10.5:1001 -> 10.10.10.10:8000
10.10.10.5:1001 -> 10.10.10.10:8000
10.10.10.10:8000 -> 10.10.10.5:1001

10.10.10.6:1001 -> 10.10.10.10:8000
10.10.10.6:1001 -> 10.10.10.10:8000
10.10.10.10:8000 -> 10.10.10.6:1001

10.10.10.5:1000 -> 10.10.10.10:8000
10.10.10.5:1000 -> 10.10.10.10:8000
10.10.10.10:8000 -> 10.10.10.5:1000
10.10.10.10:8000 -> 10.10.10.5:1000
10.10.10.10:8000 -> 10.10.10.5:1000

10.10.10.6:1000 -> 10.10.10.10:8000
10.10.10.6:1000 -> 10.10.10.10:8000
10.10.10.10:8000 -> 10.10.10.6:1000

10.10.10.7:1000 -> 10.10.10.10:8000

[download]

Packing the date might now be overkill. The following should be fine: 

while (<DATA>) {
   my ($src, $dst) = (split)[0, 2];
   my $key = $src lt $dst ? "$src $dst" : "$dst $src";
   push(@{$conversations{$key}}, $_);
}

[download]

In reply to Re^6: Sorting out troubles with advanced-ish sort by ikegami
in thread Sorting out troubles with advanced-ish sort by chargrill

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