I have a vague preference for the last one, but only because it's most like how I'd actually write it:

my @filtered = do for $s.comb -> $a, $b {
   [ $a, $b ] if $a ne $b;
}
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or maybe slightly clearer would be:

my @filtered = gather for $s.comb -> $a, $b {
   take [ $a, $b ] if $a ne $b;
}
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But mathematicians are likely to prefer the list comprehension syntax, I suspect.