Ha! Thanks for the confirmation. It is a strange one. Following on from your use an array solution, I tried the ol' put it in a list context trick--which is probably a erroneous name, but it serves-- and voila. That also fixes it. Maybe an interation between two different optimisations? Time for a perlbug I think.

my @r;
reduce{
    print "$a:$b";
    push @r, [ $a, $b ];
    $b;
} () = split ' ', $s;
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